MAT1A and disorder of methionine catabolism: Also, by expressing the mutated allele in bacteria cells, the relative activity of the p.Arg199His isoform has been proven to be about 11% of wild-type MAT activity (Chamberlin et al., 1996), although the c.596G>A carriers do not usually display hypermethioninemia (Chien et al., 2015).