Thus, with the biochemical characterization of the recombinant MOCOS proteins carrying amino acid substitutions, as found in families F1, F3 and F8 (Thr349I), case G1 (Pro591Ser) and the case described by Peretz et al. [13], Arg776Cys in the present work for the first time provides evidence for the biochemical and functional basis of type II xanthinuria. The gene discussed is MOCOS; the disease is hereditary xanthinuria.