Defects in the PSAP gene can cause a deficiency of either the entire pSap protein (prosaposin deficiency, pSap-d) or an individual Sap: SapA-d, SapB-d, SapC-d, or SapD-d, with, to date, SapD-d only being reported in an animal model [Matsuda et al., 2004]. This evidence concerns the gene PSAP and hyperinsulinemic hypoglycemia, familial, 4.