HPRTMunich is a mutant form of human hypoxanthine-guanine phosphoribosyltransferase that was isolated from a patient who presented with gout and a partial deficiency of enzyme activity. Profound abnormalities in the catalytic function of HPRTMunich are responsible for the deficiency of enzyme activity in vivo. Tryptic peptides of HPRTMunich were mapped by reverse phase high pressure liquid chromatography in an attempt to define the precise abnormality in its primary structure. Sequence analysis of aberrant peptides localized the structural alteration in HPRTMunich to residue 103. Several additional findings suggest that the mutation in this variant is most likely a serine to arginine substitution at residue 103. This amino acid substitution lies within the putative hypoxanthine-binding site of human hypoxanthine-guanine phosphoribosyltransferase possibly explaining its selective effect on intrinsic enzyme activity and binding of hypoxanthine.