In the current study, 110 cases of glucose-6-phosphate dehydrogenase deficiency were studied at the molecular level to evaluate the molecular heterogeneity of this deficiency and the frequency of the silent C/T polymorphism at nt 1311 in glucose-6-phosphate dehydrogenase deficient subjects in the Jeddah area. The gene discussed is G6PD; the disease is hyperinsulinemic hypoglycemia, familial, 4.