Glucose 6-phosphate dehydrogenase (G6PD) is a cytosolic enzyme encoded by a 
housekeeping X-linked gene whose main function is to produce NADPH, a key 
electron donor in the defense against oxidizing agents and in reductive 
biosynthetic reactions. Inherited G6PD deficiency is associated with either 
episodic hemolytic anemia (triggered by fava beans or other agents) or life-long 
hemolytic anemia. We show here that an evolutionary analysis is a key to 
understanding the biology of a housekeeping gene. From the alignment of the 
amino acid (aa) sequence of 52 glucose 6-phosphate dehydrogenase (G6PD) species 
from 42 different organisms, we found a striking correlation between the aa 
replacements that cause G6PD deficiency in humans and the sequence conservation 
of G6PD: two-thirds of such replacements are in highly and moderately conserved 
(50-99%) aa; relatively few are in fully conserved aa (where they might be 
lethal) or in poorly conserved aa, where presumably they simply would not cause 
G6PD deficiency. This is consistent with the notion that all human mutants have 
residual enzyme activity and that null mutations are lethal at some stage of 
development. Comparing the distribution of mutations in a human housekeeping 
gene with evolutionary conservation is a useful tool for pinpointing amino acid 
residues important for the stability or the function of the corresponding 
protein. In view of the current explosive increase in full genome sequencing 
projects, this tool will become rapidly available for numerous other genes.