xanthine phosphoribosyltransferase; Xan phosphoribosyltransferase; xanthosine 5'-phosphate pyrophosphorylase; xanthylate pyrophosphorylase; xanthylic pyrophosphorylase; XMP pyrophosphorylase; 5-phospho-alpha-D-ribose-1-diphosphate:xanthine phospho-D • • •