Glucose 6-phosphate dehydrogenase (G6PD) plays a key role in the generation of NADPH which is essential for maintaining glutathione in the reduce state, and in the production of ribose 5-phosphate for the synthesis of nucleotides. G6PD in its active form is either a dimer or tetramer consisting identical subunits. The gene for G6PD is located on the X chromosome. Human red cell G6PD consists of 515 amino acids with a molecular weight of 59,265 daltons. Deficiency of G6PD is the most common metabolic disorder, and is associated with chronic and drug- or infection-induced hemolytic anemia. It is estimated that 400 million people in the world are affected. The mutations responsible for about 73 variants have been determined. Some of them have polymorphic frequencies in different populations. Except for seven kinds of variants with small gene deletion, splice site deletion of intron or three nucleotide substitutions, all of those were found to be produced by one or two nucleotide substitutions. Molecular studies disclosed that all the class 1 variants associated with chronic hemolysis have the mutations surrounding either the substrate or the NADP binding site.