The same extra FokI cleavage site exists in glucose-6-phosphate dehydrogenase variants A(+) and A(-).

Two common variants of glucose-6-phosphate dehydrogenase (G6PD), i.e., A(+) and A(-), exist in blacks in high frequencies. The mutation of the A(+) gene is a single nucleotide transition, A/G in equilibrium Asp) in the G6PD protein and produces an additional FokI cleavage site of the mutation site. Thus, the FokI fragment types detected by the genomic clone that contain the mutation site differ in the normal B(+) DNA and the variant A(+) DNA. The FokI fragment type of the variant A(-) is the same as that of the A(+). Since A(+) and A(-) enzymes differ at the protein level, the A(-) gene was presumably evolved by stepwise mutations through the A(+) gene.