In addition, biochemical abnormalities in ARSA activity and/or sulfatides that mimic those in MLD are found in individuals with multiple sulfatase deficiency (MSD; MIM #272200) and saposin B deficiency (MIM #249900), ultrarare disorders due to biallelic variants in different genes, SUMF1 (the gene encoding formylglycine-generating enzyme) and PSAP (the gene encoding prosaposin), respectively [1,8,73]. Here, PSAP is linked to hyperinsulinemic hypoglycemia, familial, 4.