Common to many other metabolic disorders (e.g., phenylketonuria (PKU) [Mitchell et al., 2011]; ornithine transcarbamylase deficiency [Shchelochkov et al., 2009]), the largest proportion of MUT mutations are missense changes (131 of 243, 54%; hgmd.org) whose effects on the protein are difficult to predict a priori [Froese and Gravel, 2010; Yue et al., 2014]. This evidence concerns the gene MMUT and metabolic disease.