MLD is caused by the deficiency of arylsulfatase A lysosomal enzyme (ARSA) (OMIM 250100) and sphingolipid activator protein B (SapB or saposin B) (OMIM 249900) as a consequence of mutations in the ARSA and PSAP genes, respectively. Here, ARSA is linked to metachromatic leukodystrophy.