Cri-du-chat syndrome was first described by Lejeune et al. (1963) as a hereditary congenital syndrome associated with deletion of part of the short arm of chromosome 5. The deletions can vary in size from extremely small and involving only band 5p15.2 to the entire short arm. Although the majority of deletions arise as new mutations, approximately 12% result from unbalanced segregation of translocations or recombination involving a pericentric inversion in one of the parents.

Orofaciodigital syndrome V (OFD5) is an autosomal recessive disorder characterized by cleft palate/uvula, lobulated tongue, frontal bossing, hypertelorism, postaxial polydactyly, and impaired intellectual development (summary by Faily et al., 2017).

White-Sutton syndrome is a neurodevelopmental disorder characterized by a wide spectrum of cognitive dysfunction, developmental delays (particularly in speech and language acquisition), hypotonia, autism spectrum disorder, and other behavioral problems. Additional features commonly reported include seizures, refractive errors and strabismus, hearing loss, sleep disturbance (particularly sleep apnea), feeding and gastrointestinal problems, mild genital abnormalities in males, and urinary tract involvement in both males and females.

Glycosylphosphatidylinositol biosynthesis defect-17 (GPIBD17) is an autosomal recessive disorder characterized by variable neurologic deficits that become apparent in infancy or early childhood. Patients may present with early-onset febrile or afebrile seizures that tend to be mild or controllable. Other features may include learning disabilities, autism, behavioral abnormalities, hypotonia, and motor deficits. The phenotype is relatively mild compared to that of other GPIBDs (summary by Nguyen et al., 2018). For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Blepharophimosis-impaired intellectual development syndrome (BIS) is a congenital disorder characterized by a distinct facial appearance with blepharophimosis and global development delay. Affected individuals have delayed motor skills, sometimes with inability to walk, and impaired intellectual development with poor or absent speech; some patients show behavioral abnormalities. There are recognizable facial features, including epicanthal folds, sparse eyebrows, broad nasal bridge, short nose with downturned tip, and open mouth with thin upper lip. Other more variable features include distal skeletal anomalies, feeding difficulties with poor growth, respiratory infections, and hypotonia with peripheral spasticity (summary by Cappuccio et al., 2020).

Neurodevelopmental disorder with hypotonia, impaired language, and dysmorphic features (NEDHILD) is a rare neurodevelopmental disorder associated with impaired intellectual development, speech and language impairment, microcephaly, seizures, hypotonia, ophthalmologic issues, constipation/gastroesophageal reflux, and behavioral problems, including autism and sleep disturbances (summary by Garrity et al., 2021).

Autosomal dominant intellectual developmental disorder-72 (MRD72) is characterized by developmental delay, predominant speech delay, autistic or attention-deficit/hyperactivity disorder features, overfriendliness, generalized hypotonia, overweight/obesity, and dysmorphic features (Cuinat et al., 2022).