Alternative treatments that are being explored for PNP deficiency include injections of a PNP enzyme replacement with human PNP fused to a protein transduction domain (TATPNP) (24) or gene therapy with the PNP gene delivered ex-vivo into autologous PNP-deficient HSC (25). The gene discussed is PNP; the disease is hyperinsulinemic hypoglycemia, familial, 4.