For three patients with the Lesch-Nyhan syndrome the existence of normal amounts 
of catalytically inactive hypoxanthine-guanine phosphoribosyltransferase (HGPRT) 
protein was demonstrated by using antibodies against the normal enzyme subunits. 
The lack of enzyme activity is reverted in virus transformed cells. Individual 
revertant cell clones contain different HGPRT enzymes as demonstrated here by 
isoelectric focusing. The data strongly support the idea of a structural gene 
mutation as the cause of enzyme deficiency in the Lesch-Nyhan syndrome.