We review here some recent data about Glucose-6-phosphate dehydrogenase (G6PD), 
the housekeeping X-linked gene encoding the first enzyme of the pentose 
phosphate pathway (PPP), a NADPH-producing dehydrogenase. This enzyme has been 
popular among clinicians, biochemists, geneticists and molecular biologists 
because it is the most common form of red blood cell enzymopathy. G6PD deficient 
erythrocytes do not generate NADPH in any other way than through the PPP and for 
this reason they are more susceptible than any other cells to oxidative damage. 
Moreover, this enzyme has also been of crucial importance in many significant 
discoveries; indeed, G6PD polymorphisms have been instrumental in studying 
X-inactivation in the human species, as well as in establishing the clonal 
nature of certain tumors. G6PD deficiency, generally considered as a mild and 
benign condition, is significantly disadvantageous in certain environmental 
conditions like in presence of certain drugs. Nevertheless, G6PD deficiency has 
been positively selected by malaria, and recent knowledge seems to show that it 
also confers an advantage against the development of cancer, reduces the risk of 
coronary diseases and has a beneficial effect in terms of longevity.