Participants include 15 patients (11 females; aged 7–31 years) from 12 unrelated Sicilian families, with proven MPS III diagnosis: MPS III A (N-sulfoglucosamine sulfohydrolase deficiency) (n = 7, 47%), III B (N-acetyl-glucosaminidase deficiency) (n = 5, 34%), MPS III C (n = 2, 13%) (heparan-alpha-glucosaminide N-acetyltransferase deficiency), and MPS III D (n = 1, 6%) (N-acetylglucosamine-6-sulfatase deficiency) (Table 1). Here, HGSNAT is linked to hyperinsulinemic hypoglycemia, familial, 4.