Hypomyelinating leukodystrophy-20 (HLD20) is an autosomal recessive neurodegenerative disorder characterized by the loss of developmental milestones at about 12 to 16 months of age after normal early development. Patients lose motor, language, and cognitive skills and show poor overall growth with microcephaly. The disorder is progressive, resulting in feeding difficulties and spastic quadriplegia. Some patients may have seizures. Brain imaging shows subcortical white matter abnormalities and a thin corpus callosum, suggesting a myelination defect. Death usually occurs in childhood (Al-Abdi et al., 2020). For a discussion of genetic heterogeneity of HLD, see 312080.