G6PD deficiency: Deficiency of the enzyme glucose-6-phosphate dehydrogenase (G6PD), the most common enzyme defect of medical importance. About 10 percent of American black males have G6PD deficiency, as do a lesser percent of black females. G6PD deficiency is also increased in frequency in people of Mediterranean origin (including Italians, Greeks, Arabs, and Jews). The gene encoding G6PD is on the X chromosome. Males with this enzyme deficiency may develop anemia due to the breakup of their red blood cells when they are exposed to oxidant drugs, naphthalene moth balls, or fava beans. The offending drugs include the antimalarial primaquine, salicylates, sulfonamide antibiotics, nitrofurans, phenacetin, and some vitamin K derivatives. Fever, viral and bacterial infections, and diabetic acidosis can also precipitate a hemolytic crisis (when the red blood cells break up), resulting in anemia and jaundice. The concentration of G6PD deficiency in certain populations is believed to reflect a protective effect it afforded (much like sickle cell trait) against malaria.