When screen-positive patients are found by biochemical tests, differential diagnosis with other MLD-mimicking disorders is required, including prosaposin B deficiency due to bi-allelic variants in prosaposine gene (PSAP) and multiple sulfatase deficiency (MSD) due to bi-allelic variants in the sulfatase modifying factor 1 gene (SUMF1). The gene discussed is SUMF1; the disease is hyperinsulinemic hypoglycemia, familial, 4.