S-adenosylmethionine (AdoMet, SAM) is an important methyl donor in most transmethylation reactions. S-adenosylmethionine synthase isoform type-1 (MAT1A) catalyses the formation of AdoMet from methionine and ATP. Defects in MAT1A can cause methionine adenosyltransferase deficiency (MATD; MIM:250850), an inborn error of metabolism resulting in hypermethioninemia. In this condition, methionine accumulates because its conversion to AdoMet is impaired. Frameshift mutations causing complete loss of MAT1A function are K181Vfs*5, H277Afs*75 and V348Gfs*3 (Hazelwood et al. 1998, Chamberlin et al. 1996).