Adenosylhomocysteinase (AHCY) is a tetrameric, NAD+-bound, cytosolic protein that regulates all adenosylmethionine-(AdoMet) dependent transmethylations by hydrolysing the feedback inhibitor adenosylhomocysteine (AdoHcy) to homocysteine (HCYS) and adenosine (Ade-Rib) (Turner et al. 1998, Yang et al. 2003). Defects in AHCY cause Hypermethioninemia with S-adenosylhomocysteine hydrolase deficiency (HMAHCHD; MIM:613752), a metabolic disorder characterised by hypermethioninemia associated with failure to thrive, psychomotor retardation, facial dysmorphism with abnormal hair and teeth and myocardiopathy. Gene analysis revealed two mutations that can cause HMAHCHD; W112* and Y143C (Baric et al. 2004).