Glucose-6-phosphate dehydrogenase (G6PD): Deficiency of G6PD is the commonest disease-causing enzyme defect in humans affecting an estimated 400 million people. The G6PD gene is on the X chromosome. Males with the enzyme deficiency develop anemia due to breakup of their red blood cells when they are exposed to oxidant drugs such as the antimalarial primaquine, the sulfonamide antibiotics or sulfones, naphthalene moth balls, or fava beans.