An inherited disorder transmitted as an X-linked (sex-linked) recessive trait. It is present in the U.S. in about 13% of black males and 2% of black females. The enzyme deficiency also occurs in Arabic, Mediterranean, and Asian ethnic groups. The enzyme is essential to maintaining the integrity of red blood cells; thus a deficiency of it causes nonimmune hemolytic anemia. There are many variants of the enzyme and great variation in severity of the disease. Some individuals do not have clinical symptoms until they are exposed to certain drugs (e.g., antimalarials, antipyretics, sulfonamides) or to fava beans, or when they contract an infectious disease. In others the condition is present at birth. When present at birth, anemia, hepatomegaly, hypoglycemia, and interference with growth are present. In those who have the deficiency but are not affected until exposed to certain drugs or infections, hemolytic anemia and jaundice occur.