ARSA and metachromatic leukodystrophy: This process underlies several types of lysosomal storage disorders, including MLD and Gaucher disease,20 which are characterized by neurodegeneration and other systemic symptoms.21PSAP gene mutations, particularly those affecting the coding region within the B domain of saposin, can lead to sulfate accumulation and result in rare MLD variants with normal arylsulfatase A activity.22 Saposins all originate from the same precursor, prosaposin, in the late endosome/lysosome and can regulate the lysosomal enzymatic degradation of various sphingolipids.