Smith-Lemli-Opitz syndrome (SLOS) is a congenital multiple-anomaly / cognitive impairment syndrome caused by an abnormality in cholesterol metabolism resulting from deficiency of the enzyme 7-dehydrocholesterol (7-DHC) reductase. It is characterized by prenatal and postnatal growth restriction, microcephaly, moderate-to-severe intellectual disability, and multiple major and minor malformations. The malformations include distinctive facial features, cleft palate, cardiac defects, underdeveloped external genitalia in males, postaxial polydactyly, and 2-3 syndactyly of the toes. The clinical spectrum is wide; individuals with normal development and only minor malformations have been described.

Nager syndrome is the prototype for a group of disorders collectively referred to as the acrofacial dysostoses (AFDs), which are characterized by malformation of the craniofacial skeleton and the limbs. The major facial features of Nager syndrome include downslanted palpebral fissures, midface retrusion, and micrognathia, the latter of which often requires the placement of a tracheostomy in early childhood. Limb defects typically involve the anterior (radial) elements of the upper limbs and manifest as small or absent thumbs, triphalangeal thumbs, radial hypoplasia or aplasia, and radioulnar synostosis. Phocomelia of the upper limbs and, occasionally, lower-limb defects have also been reported. The presence of anterior upper-limb defects and the typical lack of lower-limb involvement distinguishes Nager syndrome from Miller syndrome (263750), another rare AFD; however, distinguishing Nager syndrome from other AFDs, including Miller syndrome, can be challenging (summary by Bernier et al., 2012).

The phenotypic spectrum of SHOX deficiency disorders, caused by haploinsufficiency of the short stature homeobox-containing gene (SHOX), ranges from Leri-Weill dyschondrosteosis (LWD) at the severe end of the spectrum to nonspecific short stature at the mild end of the spectrum. In adults with SHOX deficiency, the proportion of LWD versus short stature without features of LWD is not well defined. In LWD the classic clinical triad is short stature, mesomelia, and Madelung deformity. Mesomelia, in which the middle portion of a limb is shortened in relation to the proximal portion, can be evident first in school-aged children and increases with age in frequency and severity. Madelung deformity (abnormal alignment of the radius, ulna, and carpal bones at the wrist) typically develops in mid-to-late childhood and is more common and severe in females. The phenotype of short stature caused by SHOX deficiency in the absence of mesomelia and Madelung deformity (called SHOX-deficient short stature in this GeneReview) is highly variable, even within the same family.

Borjeson-Forssman-Lehmann syndrome (BFLS) is an uncommon X-linked intellectual developmental disorder that evolves with age. Clinical manifestations in males are quite variable, with the most consistent features being initial hypotonia, mild to moderate impaired intellectual development, large fleshy ears, underdeveloped genitalia, gynecomastia, truncal obesity, tapering fingers, and shortening of the fourth and fifth toes. Heterozygous females may have a milder similar clinical phenotype, which can include hypothyroidism; however, many carrier females appear unaffected (summary by Crawford et al., 2006).

Pallister-Killian syndrome (PKS) is a dysmorphic condition involving most organ systems, but is also characterized by a tissue-limited mosaicism; most fibroblasts have 47 chromosomes with an extra small metacentric chromosome, whereas the karyotype of lymphocytes is normal. The extra metacentric chromosome is an isochromosome for part of the short arm of chromosome 12: i(12)(p10) (Peltomaki et al., 1987; Warburton et al., 1987).

Short-rib thoracic dysplasia (SRTD) with or without polydactyly refers to a group of autosomal recessive skeletal ciliopathies that are characterized by a constricted thoracic cage, short ribs, shortened tubular bones, and a 'trident' appearance of the acetabular roof. SRTD encompasses Ellis-van Creveld syndrome (EVC) and the disorders previously designated as Jeune syndrome or asphyxiating thoracic dystrophy (ATD), short rib-polydactyly syndrome (SRPS), and Mainzer-Saldino syndrome (MZSDS). Polydactyly is variably present, and there is phenotypic overlap in the various forms of SRTDs, which differ by visceral malformation and metaphyseal appearance. Nonskeletal involvement can include cleft lip/palate as well as anomalies of major organs such as the brain, eye, heart, kidneys, liver, pancreas, intestines, and genitalia. Some forms of SRTD are lethal in the neonatal period due to respiratory insufficiency secondary to a severely restricted thoracic cage, whereas others are compatible with life (summary by Huber and Cormier-Daire, 2012 and Schmidts et al., 2013). There is phenotypic overlap with the cranioectodermal dysplasias (Sensenbrenner syndrome; see CED1, 218330). Patients with a clinical diagnosis of Beemer-Langer syndrome have been found to carry mutations in the IFT80 gene (611177); see SRTD2, 611263. For a discussion of genetic heterogeneity of short-rib thoracic dysplasia, see SRTD1 (208500).

Cranioectodermal dysplasia (CED) is a ciliopathy with skeletal involvement (narrow thorax, shortened proximal limbs, syndactyly, polydactyly, brachydactyly), ectodermal features (widely spaced hypoplastic teeth, hypodontia, sparse hair, skin laxity, abnormal nails), joint laxity, growth deficiency, and characteristic facial features (frontal bossing, low-set simple ears, high forehead, telecanthus, epicanthal folds, full cheeks, everted lower lip). Most affected children develop nephronophthisis that often leads to end-stage kidney disease in infancy or childhood, a major cause of morbidity and mortality. Hepatic fibrosis and retinal dystrophy are also observed. Dolichocephaly, often secondary to sagittal craniosynostosis, is a primary manifestation that distinguishes CED from most other ciliopathies. Brain malformations and developmental delay may also occur.

Peters plus syndrome is characterized by anterior chamber eye anomalies, short limbs with broad distal extremities, characteristic facial features, cleft lip/palate, and variable developmental delay / intellectual disability. The most common anterior chamber defect is Peters' anomaly, consisting of central corneal clouding, thinning of the posterior cornea, and iridocorneal adhesions. Cataracts and glaucoma are common. Developmental delay is observed in about 80% of children; intellectual disability can range from mild to severe.

Myhre syndrome is a connective tissue disorder with multisystem involvement, progressive and proliferative fibrosis that may occur spontaneously or following trauma or surgery, mild-to-moderate intellectual disability, and in some instances, autistic-like behaviors. Organ systems primarily involved include: cardiovascular (congenital heart defects, long- and short-segment stenosis of the aorta and peripheral arteries, pericardial effusion, constrictive pericarditis, restrictive cardiomyopathy, and hypertension); respiratory (choanal stenosis, laryngotracheal narrowing, obstructive airway disease, or restrictive pulmonary disease), gastrointestinal (pyloric stenosis, duodenal strictures, severe constipation); and skin (thickened particularly on the hands and extensor surfaces). Additional findings include distinctive craniofacial features and skeletal involvement (intrauterine growth restriction, short stature, limited joint range of motion). To date, 55 individuals with molecularly confirmed Myhre syndrome have been reported.

Spondyloperipheral dysplasia is a disorder that impairs bone growth. This condition is characterized by flattened bones of the spine (platyspondyly) and unusually short fingers and toes (brachydactyly), with the exception of the first (big) toes. Other skeletal abnormalities associated with spondyloperipheral dysplasia include short stature, shortened long bones of the arms and legs, exaggerated curvature of the lower back (lordosis), and an inward- and upward-turning foot (clubfoot). Additionally, some affected individuals have nearsightedness (myopia), hearing loss, and intellectual disability.

Bohring-Opitz syndrome (BOS) is characterized by distinctive facial features and posture, growth failure, variable but usually severe intellectual disability, and variable anomalies. The facial features may include microcephaly or trigonocephaly / prominent (but not fused) metopic ridge, hypotonic facies with full cheeks, synophrys, glabellar and eyelid nevus flammeus (simplex), prominent globes, widely set eyes, palate anomalies, and micrognathia. The BOS posture, which is most striking in early childhood and often becomes less apparent with age, is characterized by flexion at the elbows with ulnar deviation and flexion of the wrists and metacarpophalangeal joints. Feeding difficulties in early childhood, including cyclic vomiting, have a significant impact on overall health; feeding tends to improve with age. Seizures are common and typically responsive to standard epileptic medications. Minor cardiac anomalies and transient bradycardia and apnea may be present. Affected individuals may experience recurrent infections, which also tend to improve with age. Isolated case reports suggest that individuals with BOS are at greater risk for Wilms tumor than the general population, but large-scale epidemiologic studies have not been conducted.

Czech dysplasia is an autosomal dominant skeletal dysplasia characterized by early-onset, progressive pseudorheumatoid arthritis, platyspondyly, and short third and fourth toes (Marik et al., 2004; Kozlowski et al., 2004).

Catel-Manzke syndrome is characterized by the Pierre Robin anomaly, which comprises cleft palate, glossoptosis, and micrognathia, and a unique form of bilateral hyperphalangy in which there is an accessory bone inserted between the second metacarpal and its corresponding proximal phalanx, resulting in radial deviation of the index finger (summary by Manzke et al., 2008).

Roifman syndrome is a multisystem disorder characterized by growth retardation, spondyloepiphyseal dysplasia, retinal dystrophy, distinctive facial dysmorphism, and immunodeficiency (summary by de Vries et al., 2006).

Terminal osseous dysplasia is an X-linked dominant male-lethal disease characterized by skeletal dysplasia of the limbs, pigmentary defects of the skin, and recurrent digital fibroma during infancy (Sun et al., 2010).

Short-rib thoracic dysplasia (SRTD) with or without polydactyly refers to a group of autosomal recessive skeletal ciliopathies that are characterized by a constricted thoracic cage, short ribs, shortened tubular bones, and a 'trident' appearance of the acetabular roof. SRTD encompasses Ellis-van Creveld syndrome (EVC) and the disorders previously designated as Jeune syndrome or asphyxiating thoracic dystrophy (ATD), short rib-polydactyly syndrome (SRPS), and Mainzer-Saldino syndrome (MZSDS). Polydactyly is variably present, and there is phenotypic overlap in the various forms of SRTDs, which differ by visceral malformation and metaphyseal appearance. Nonskeletal involvement can include cleft lip/palate as well as anomalies of major organs such as the brain, eye, heart, kidneys, liver, pancreas, intestines, and genitalia. Some forms of SRTD are lethal in the neonatal period due to respiratory insufficiency secondary to a severely restricted thoracic cage, whereas others are compatible with life (summary by Huber and Cormier-Daire, 2012 and Schmidts et al., 2013). There is phenotypic overlap with the cranioectodermal dysplasias (Sensenbrenner syndrome; see CED1, 218330). For a discussion of genetic heterogeneity of short-rib thoracic dysplasia, see SRTD1 (208500).

Wiedemann-Steiner syndrome (WSS) is characterized by developmental delay, intellectual disability, and characteristic facial features, with or without additional congenital anomalies. The facial features include thick eyebrows with lateral flare, vertically narrow and downslanted palpebral fissures, widely spaced eyes, long eyelashes, wide nasal bridge, broad nasal tip, thin vermilion of the upper lip, and thick scalp hair. About 60% of affected individuals have hypertrichosis cubiti ("hairy elbows"), which was once thought to be pathognomic for the syndrome, with a majority having hypertrichosis of other body parts. Other clinical features include feeding difficulties, prenatal and postnatal growth restriction, epilepsy, ophthalmologic anomalies, congenital heart defects, hand anomalies (such as brachydactyly and clinodactyly), hypotonia, vertebral anomalies (especially fusion anomalies of the cervical spine), renal and uterine anomalies, immune dysfunction, brain malformations, and dental anomalies.

Sedaghatian-type spondylometaphyseal dysplasia (SMDS) is a rare lethal disorder characterized by severe metaphyseal chondrodysplasia with mild limb shortening, platyspondyly, delayed epiphyseal ossification, irregular iliac crests, and pulmonary hemorrhage. Affected infants present with severe hypotonia and cardiorespiratory problems; most die within days of birth due to respiratory failure. Cardiac abnormalities include conduction defects, complete heart block, and structural anomalies. Half of infants with SMDS are reported to have central nervous system malformations consistent with abnormal neuronal migration, including agenesis of the corpus callosum, pronounced frontotemporal pachygyria, simplified gyral pattern, partial lissencephaly, and severe cerebellar hypoplasia (summary by Smith et al., 2014).

Yunis-Varon syndrome (YVS) is a severe autosomal recessive disorder characterized by skeletal defects, including cleidocranial dysplasia and digital anomalies, and severe neurologic involvement with neuronal loss. Enlarged cytoplasmic vacuoles are found in neurons, muscle, and cartilage. The disorder is usually lethal in infancy (summary by Campeau et al., 2013).

Patent ductus arteriosus - bicuspid aortic valve - hand anomalies syndrome is a very rare heart-hand syndrome (see this term) that is characterized by a variety of cardiovascular anomalies including patent arterial duct, bicuspid aortic valve and pseudocoarctation of the aorta in conjunction with hand anomalies such as brachydactyly and ulnar ray derivative i.e. fifth metacarpal hypoplasia. Transmission is most likely autosomal dominant.

An extremely rare brachydactyly syndrome with characteristics of short broad hands and feet with brachydactyly associated with congenital flexion contractures of the proximal and/or distal interphalangeal joints of the fingers, as well as syndactyly of feet. Polydactyly, septate vagina and urinary incontinence were also occasionally reported. Camptobrachydactyly has been described in 18 members of 1 family, suggesting an autosomal dominant inheritance. There have been no further descriptions in the literature since 1972.

The Osebold-Remondini syndrome is a bone dysplasia with mesomelic shortness of limbs and, hence, shortness of stature, absence or hypoplasia of second phalanges with synostosis of the remaining phalanges, carpal and tarsal coalitions, and apparently no other anomalies (summary by Opitz and Gilbert, 1985). See 602875 for a discussion of genetic heterogeneity of autosomal recessive acromesomelic dysplasia.

The acromesomelic dysplasias are disorders in which there is disproportionate shortening of skeletal elements, predominantly affecting the middle segments (forearms and forelegs) and distal segments (hands and feet) of the appendicular skeleton. Acromesomelic dysplasia-1 (AMD1) is characterized by severe dwarfism (height below 120 cm) with shortening of the middle and distal segments of the limbs. This condition is usually diagnosed at birth and becomes more obvious in the first 2 years of life. X-rays show short broad fingers, square flat feet, and shortening of the long bones (particularly the forearms). The radius is bowed; the ulna is shorter than the radius, and its distal end is occasionally hypoplastic. The skull is dolichocephalic and a shortness of the trunk, with decreased vertebral height and narrowing of the lumbar interpedicular distances, is consistently observed. Facial appearance and intelligence are normal (summary by Faivre et al., 2000). Genetic Heterogeneity of Acromesomelic Dysplasia Additional autosomal recessive forms of acromesomelic dysplasia include acromesomelic dysplasia-2A (200700), -2B (228900), and -2C (201250), all caused by mutation in the GDF5 gene (601146) on chromosome 20q11; AMD3 (200700), caused by mutation in the BMPR1B gene (603248) on chromosome 4q22; and AMD4 (619636), caused by mutation in the PRKG2 gene (601591) on chromosome 4q21. An autosomal dominant form of acromesomelic dysplasia has also been reported (see 112910).

Pierpont syndrome (PRPTS) is a multiple congenital anomaly syndrome associated with learning disability. Key features include distinctive facial characteristics, especially when smiling, plantar fat pads, and other limb anomalies (summary by Burkitt Wright et al., 2011).

An extremely rare type of spondyloepiphyseal dysplasia described in about 5 patients to date with clinical signs including short stature, peculiar facies with blepharophimosis, upward slanted eyes, abundant eyebrows and eyelashes, coarse voice, and short hands and feet.

Any autosomal dominant non-syndromic intellectual disability in which the cause of the disease is a mutation in the KIRREL3 gene.

Dermatosparaxis (meaning 'tearing of skin') is an autosomal recessive disorder of connective tissue resulting from deficiency of procollagen peptidase, an enzyme that aids in the processing of type I procollagen. The disorder and the responsible biochemical defect was first observed in cattle (Lapiere et al., 1971). Lapiere and Nusgens (1993) reviewed the discovery of dermatosparaxis in cattle, the elucidation of the disorder, its occurrence in other animals, and the delayed recognition of the disorder in the human.

Patients with chromosome 2q37 deletion syndrome show highly variable clinical manifestations likely resulting from different deletion sizes and deletions of different genes. Variable clinical features included brachydactyly type E (BDE), affecting the metacarpals and metatarsals (in about 50% of patients), short stature, mild to moderate intellectual disability, behavioral abnormalities, and dysmorphic facial features. However, many individuals with deletions do not show cognitive deficits (summary by Villavicencio-Lorini et al., 2013, Wheeler et al., 2014, Jean-Marcais et al., 2015).

16p13.3 microduplication syndrome is a rare chromosomal anomaly syndrome resulting from a partial duplication of the short arm of chromosome 16 and manifesting with a variable phenotype which is mostly characterized by: mild to moderate intellectual deficit and developmental delay (particularly speech), normal growth, short, proximally implanted thumbs and other hand and feet malformations (such as camptodactyly, syndactyly, club feet), mild arthrogryposis and characteristic facies (upslanting, narrow palpebral fissures, hypertelorism, mid face hypoplasia, bulbous nasal tip and low set ears). Other reported manifestations include cryptorchidism, inguinal hernia and behavioral problems.

The GPAPP-type of chondrodysplasia with joint dislocations is an autosomal recessive disorder characterized by short stature, chondrodysplasia with brachydactyly, congenital joint dislocations, cleft palate, and facial dysmorphism (Vissers et al., 2011).

Disorders of GNAS inactivation include the phenotypes pseudohypoparathyroidism Ia, Ib, and Ic (PHP-Ia, -Ib, -Ic), pseudopseudohypoparathyroidism (PPHP), progressive osseous heteroplasia (POH), and osteoma cutis (OC). PHP-Ia and PHP-Ic are characterized by: End-organ resistance to endocrine hormones including parathyroid hormone (PTH), thyroid-stimulating hormone (TSH), gonadotropins (LH and FSH), growth hormone-releasing hormone (GHRH), and CNS neurotransmitters (leading to obesity and variable degrees of intellectual disability and developmental delay); and The Albright hereditary osteodystrophy (AHO) phenotype (short stature, round facies, and subcutaneous ossifications) and brachydactyly type E (shortening mainly of the 4th and/or 5th metacarpals and metatarsals and distal phalanx of the thumb). Although PHP-Ib is characterized principally by PTH resistance, some individuals also have partial TSH resistance and mild features of AHO (e.g., brachydactyly). PPHP, a more limited form of PHP-Ia, is characterized by various manifestations of the AHO phenotype without the hormone resistance or obesity. POH and OC are even more restricted variants of PPHP: POH consists of dermal ossification beginning in infancy, followed by increasing and extensive bone formation in deep muscle and fascia. OC consists of extra-skeletal ossification that is limited to the dermis and subcutaneous tissues.

Adams-Oliver syndrome (AOS) is characterized by aplasia cutis congenita (ACC) of the scalp and terminal transverse limb defects (TTLD). ACC lesions usually occur in the midline of the parietal or occipital regions, but can also occur on the abdomen or limbs. At birth, an ACC lesion may already have the appearance of a healed scar. ACC lesions less than 5 cm often involve only the skin and almost always heal over a period of months; larger lesions are more likely to involve the skull and possibly the dura, and are at greater risk for complications, which can include infection, hemorrhage, or thrombosis, and can result in death. The limb defects range from mild (unilateral or bilateral short distal phalanges) to severe (complete absence of all toes or fingers, feet or hands, or more, often resembling an amputation). The lower extremities are almost always more severely affected than the upper extremities. Additional major features frequently include cardiovascular malformations/dysfunction (23%), brain anomalies, and less frequently renal, liver, and eye anomalies.

Autosomal recessive congenital ichthyosis (ARCI) encompasses several forms of nonsyndromic ichthyosis. Although most neonates with ARCI are collodion babies, the clinical presentation and severity of ARCI may vary significantly, ranging from harlequin ichthyosis, the most severe and often fatal form, to lamellar ichthyosis (LI) and (nonbullous) congenital ichthyosiform erythroderma (CIE). These phenotypes are now recognized to fall on a continuum; however, the phenotypic descriptions are clinically useful for clarification of prognosis and management. Infants with harlequin ichthyosis are usually born prematurely and are encased in thick, hard, armor-like plates of cornified skin that severely restrict movement. Life-threatening complications in the immediate postnatal period include respiratory distress, feeding problems, and systemic infection. Collodion babies are born with a taut, shiny, translucent or opaque membrane that encases the entire body and lasts for days to weeks. LI and CIE are seemingly distinct phenotypes: classic, severe LI with dark brown, plate-like scale with no erythroderma and CIE with finer whiter scale and underlying generalized redness of the skin. Affected individuals with severe involvement can have ectropion, eclabium, scarring alopecia involving the scalp and eyebrows, and palmar and plantar keratoderma. Besides these major forms of nonsyndromic ichthyosis, a few rare subtypes have been recognized, such as bathing suit ichthyosis, self-improving collodion ichthyosis, or ichthyosis-prematurity syndrome.

Fraser syndrome is an autosomal recessive malformation disorder characterized by cryptophthalmos, syndactyly, and abnormalities of the respiratory and urogenital tract (summary by van Haelst et al., 2008). For a general phenotypic description and a discussion of genetic heterogeneity of Fraser syndrome, see 219000.

Hyperphosphatasia with impaired intellectual development syndrome-1 (HPMRS1) is an autosomal recessive disorder characterized by impaired intellectual development, various neurologic abnormalities such as seizures and hypotonia, and hyperphosphatasia. Other features include facial dysmorphism and variable degrees of brachytelephalangy (summary by Krawitz et al., 2010). The disorder is caused by a defect in glycosylphosphatidylinositol biosynthesis; see GPIBD1 (610293). Genetic Heterogeneity of Hyperphosphatasia with Impaired Intellectual Development Syndrome See also HPMRS2 (614749), caused by mutation in the PIGO gene (614730) on chromosome 9p13; HPMRS3 (614207), caused by mutation in the PGAP2 gene (615187) on chromosome 11p15; HPMRS4 (615716), caused by mutation in the PGAP3 gene (611801) on chromosome 17q12; HPMRS5 (616025), caused by mutation in the PIGW gene (610275) on chromosome 17q12; and HPMRS6 (616809), caused by mutation in the PIGY gene (610662) on chromosome 4q22. Knaus et al. (2018) provided a review of the main clinical features of the different types of HPMRS, noting that some patients have a distinct pattern of facial anomalies that can be detected by computer-assisted comparison, particularly those with mutations in the PIGV and PGAP3 genes. Individuals with HPMRS have variable increased in alkaline phosphatase (AP) as well as variable decreases in GPI-linked proteins that can be detected by flow cytometry. However, there was no clear correlation between AP levels or GPI-linked protein abnormalities and degree of neurologic involvement, mutation class, or gene involved. Knaus et al. (2018) concluded that a distinction between HPMRS and MCAHS (see, e.g., 614080), which is also caused by mutation in genes involved in GPI biosynthesis, may be artificial and even inaccurate, and that all these disorders should be considered and classified under the more encompassing term of 'GPI biosynthesis defects' (GPIBD).

Feingold syndrome 1 (referred to as FS1 in this GeneReview) is characterized by digital anomalies (shortening of the 2nd and 5th middle phalanx of the hand, clinodactyly of the 5th finger, syndactyly of toes 2-3 and/or 4-5, thumb hypoplasia), microcephaly, facial dysmorphism (short palpebral fissures and micrognathia), gastrointestinal atresias (primarily esophageal and/or duodenal), and mild-to-moderate learning disability.

The cartilage-hair hypoplasia – anauxetic dysplasia (CHH-AD) spectrum disorders are a continuum that includes the following phenotypes: Metaphyseal dysplasia without hypotrichosis (MDWH). Cartilage-hair hypoplasia (CHH). Anauxetic dysplasia (AD). CHH-AD spectrum disorders are characterized by severe disproportionate (short-limb) short stature that is usually recognized in the newborn, and occasionally prenatally because of the short extremities. Other findings include joint hypermobility, fine silky hair, immunodeficiency, anemia, increased risk for malignancy, gastrointestinal dysfunction, and impaired spermatogenesis. The most severe phenotype, AD, has the most pronounced skeletal phenotype, may be associated with atlantoaxial subluxation in the newborn, and may include cognitive deficiency. The clinical manifestations of the CHH-AD spectrum disorders are variable, even within the same family.

Distal arthrogryposis type 2B2 (DA2B2) is characterized by congenital contractures of the distal limb joints and facial dysmorphism. Marked inter- and intrafamilial variability has been reported (summary by Daly et al., 2014). For a phenotypic description and a discussion of genetic heterogeneity of distal arthrogryposis, see 108120.

Neurodevelopmental disorder with dysmorphic facies and distal skeletal anomalies (NEDDFSA) is a global neurodevelopmental disorder with highly variable features. Patients often show poor feeding, poor overall growth, and hypotonia from early infancy, followed by mildly delayed motor development, poor language acquisition, and behavioral abnormalities. Intellectual development varies from severe with absent speech to mild with the ability to attend special schools. Common features include dysmorphic facial features with notable eye anomalies, joint hypermobility, and mild skeletal anomalies of the hands and feet (summary by Carapito et al., 2019).

RAB18 deficiency is the molecular deficit underlying both Warburg micro syndrome (characterized by eye, nervous system, and endocrine abnormalities) and Martsolf syndrome (characterized by similar – but milder – findings). To date Warburg micro syndrome comprises >96% of reported individuals with genetically defined RAB18 deficiency. The hallmark ophthalmologic findings are bilateral congenital cataracts, usually accompanied by microphthalmia, microcornea (diameter <10), and small atonic pupils. Poor vision despite early cataract surgery likely results from progressive optic atrophy and cortical visual impairment. Individuals with Warburg micro syndrome have severe to profound intellectual disability (ID); those with Martsolf syndrome have mild to moderate ID. Some individuals with RAB18 deficiency also have epilepsy. In Warburg micro syndrome, a progressive ascending spastic paraplegia typically begins with spastic diplegia and contractures during the first year, followed by upper-limb involvement leading to spastic quadriplegia after about age five years, often eventually causing breathing difficulties. In Martsolf syndrome infantile hypotonia is followed primarily by slowly progressive lower-limb spasticity. Hypogonadism – when present – manifests in both syndromes, in males as micropenis and/or cryptorchidism and in females as hypoplastic labia minora, clitoral hypoplasia, and small introitus.

Odontochondrodysplasia-2 with hearing loss and diabetes (ODCD2) is characterized by growth retardation with proportionate short stature, dentinogenesis imperfecta, sensorineural hearing loss, insulin-dependent diabetes, and mild intellectual disability (Cauwels et al., 2005; Lekszas et al., 2020). For a discussion of genetic heterogeneity of ODCD, see ODCD1 (184260).

Acromesomelic dysplasia-4 (AMD4) is characterized by disproportionate short stature due to mesomelic shortening of the limbs. Radiographic hallmarks include mild to moderate platyspondyly, moderate brachydactyly, iliac flaring, and metaphyseal alterations of the long bones that progressively increase with age (Diaz-Gonzalez et al., 2022). For a discussion of genetic heterogeneity of acromesomelic dysplasia, see AMD1 (602875).

Diamond-Blackfan anemia-21 (DBA21) is an autosomal recessive bone marrow failure syndrome that includes selective erythroid hypoplasia, anemia with transient thrombocytopenia, short stature, facial dysmorphism, limb abnormalities, cardiac defects, and intellectual disability (O'Donohue et al., 2022). For a general phenotypic description and discussion of genetic heterogeneity of Diamond-Blackfan anemia, see DBA1 (105650).