For example, carriers of hypoxanthine phosphoribosyl transferase (HPRT) deficiency (Lesch–Nyhan syndrome) are almost completely devoid of mutant cells in their blood at the age of 10 years due to selection against HPRT-deficient erythrocyte precursors [23,31]. The gene discussed is HPRT1; the disease is hyperinsulinemic hypoglycemia, familial, 4.