PSAP and metachromatic leukodystrophy: MLD is caused by variants in the ARSA gene on chromosome 22q13.33, which codes for the lysosomal enzyme ASA, or, more rarely, by variants in the PSAP gene on chromosome 10q22.1, which codes for the activator protein saposin B. Cesani et al. [15] have described 200 ARSA allele variants in 432 MLD patients from 393 families and ten PSAP allele variants found in twenty-six patients from eighteen families.