Neurodevelopmental disorder with hypotonia and cerebellar atrophy, with or without seizures (NEDHCAS) is an autosomal recessive neurodevelopmental disorder characterized by global developmental delay with variably impaired intellectual development, delayed motor skills, and poor or absent speech. Most patients develop early-onset seizures and demonstrate cerebellar ataxia or dysmetria associated with progressive cerebellar atrophy on brain imaging. The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis (summary by Nguyen et al., 2020). For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).