A megalencephalic leukoencephalopathy with subcortical cysts characterized by infantile-onset macrocephaly and later onset of motor deterioration, with ataxia and spasticity, seizures, cognitive decline of variable severity, white matter abnormalities, including swelling of the cerebral white matter and subcortical cysts that has_material_basis_in homozygous or compound heterozygous mutation in HEPACAM on chromosome 11q24.