The human X-linked gene encoding glucose 6-phosphate dehydrogenase (G6PD) is 
highly polymorphic; more than 300 G6PD variants have been identified. G6PD 
deficiency in different geographical areas appears to have arisen through 
independent mutational events, but within the same population it may also be 
heterogeneous. One example is the island of Sardinia, where careful clinical and 
biochemical studies have identified four different G6PD variants. We cloned and 
sequenced the four G6PD variants from Sardinia and found that only two mutations 
are responsible for G6PD deficiency in this area: one mutation is the cause of 
the G6PD Seattle-like phenotype, a milder form of G6PD deficiency; the other 
mutation is responsible for all forms of very severe G6PD deficiency in Sardinia 
and, possibly, in the Mediterranean.