The complete cDNA sequence of G-6-PD is now known, and the application of modern technology has made it possible to rapidly accumulate sequence data regarding G-6-PD variants. The information that has become available demonstrates that there are several different types of G-6-PD A(-) but all have in common the mutation that is characteristic of G-6-PD A(+), viz. a substitution of G for A at nucleotide 376. Moreover, the same nucleotide substitutions have been found in Spanish and Italian subjects, indicating that the G-6-PD A(-) mutation is much more common in non-African populations than had been thought. G-6-PD Mediterranean, on the other hand, seems to be more uniform than had been thought previously. A wealth of information regarding structure/function relationship of enzymes is potentially available through the study of G-6-PD variants.