The glucose-6-phosphate dehydrogenase (G6PD) gene is X-linked. There are numerous mutations that cause a deficiency of this enzyme in erythrocytes. G6PD deficiency can produce anemia, both when drugs are administered and under the stress induced by infection. Functionally severe variants cause hereditary non-spherocytic hemolytic anemia, i.e. anemia even in the absence of stress. Neonatal jaundice occurs in G6PD deficiency, but it is likely that it is largely due to impairment of liver function, rather than to hemolysis. It has been suggested that there are clinical manifestations of G6PD deficiency that are related to other tissues, but the existence of these is not well documented. Some mutations that produce G6PD deficiency in red cells exist at polymorphic frequencies. Individuals with such mutations seem to have enjoyed a selective advantage because of resistance to falciparum malaria. Different mutations, each characteristic of certain populations, are found, and have been characterized at the deoxyribonucleic acid (DNA) level. G6PD A-(202A376G) is the most common African mutation. G6PD Mediterranean(563T) is found in Southern Europe, the Middle East and in the Indian subcontinent. Several other mutations are common in Asia. Genetic variability of G6PD has played an important role in the understanding of a variety of developmental processes.