PSAP and metachromatic leukodystrophy: Pathophysiologically, deficiency of one or more saposins leads to substrate accumulation in lysosomes: for example, SAP-B deficiency causes a phenotype closely resembling metachromatic leukodystrophy (MLD); SAP-C deficiency produces a variant of GD; complete prosaposin deficiency (combined SAP-A–D) leads to widespread neurovisceral storage, multi-sphingolipid accumulation, and severe neurologic and systemic disease.