infantile cerebral and cerebellar atrophy with postnatal progressive microcephaly (MONDO_0013351, a Monarch Disease Ontology entry) is a rare, central nervous system malformation syndrome characterized by progressive microcephaly with profound motor delay and intellectual disability, associated with hypertonia, spasticity, clonus, and seizures, with brain imaging revealing severe cerebral and cerebellar atrophy, and poor myelination.