XDH and colorectal carcinoma: On the other hand, significant downregulation in CRC tumors of 5′-nucleotidase (5NUC), a plasma membrane enzyme that converts extracellular nucleotides to the corresponding nucleosides to aid their entry into the cell, and xanthine oxidase/dehydrogenase (XD), which competes for substrate with HGPRT, is consistent with elevated activity of the salvage pathway [53].