Methionine adenosyltransferase deficiency is an inborn error of metabolism resulting in isolated hypermethioninemia. Most patients have no clinical abnormalities, although some with the autosomal recessive form have have neurologic abnormalities (Mudd et al., 2003; Kim et al., 2016).

Extraoral halitosis due to methanethiol oxidase deficiency (EHMTO) is a malodor syndrome resulting from an inborn error of metabolism. Breath odor is cabbage-like and results from high levels of methanethiol and dimethylsulfide in breath. Elevated urinary excretion of dimethylsulfone (DMSO2) is diagnostic (Pol et al., 2018).