Four NUDT1 ("MTH1") proteins have been identified, generated from a single gene by use of alternative start codons (Oda et al. 1999). The shortest of these, NUDT1 p18, has been biochemically characterized and shown to catalyze hydrolysis of 8-oxo-dGTP (Sakumi et al. 1993). The longer isoforms all consist of the p18 polypeptide with aminoterminal extensions and are presumed to be active as well though they have not been experimentally characterized.