Methylmalonyl CoA mutase (MUT aka MCM) (Jansen et al. 1989) utilises adenosylcobalamin (AdoCbl) as a cofactor catalyzes interchange of a carbonylCoA group and a hydrogen atom in conversion of methylmalonyl CoA to form succinyl CoA, a precursor for the citric acid cycle. MUT has a homodimeric structure and is located in the mitochondrial matrix. Defects in MUT cause methylmalonic aciduria, mut type (MMAM; MIM:251000), an often fatal disorder of organic acid metabolism (Worgan et al. 2006). Patients with no residual MUT activity are defined as MMA mut(0); those with some residual MUT activity are defined as MMA mut(-). Mutations causing MMA mut(0) type of disorder include Q17*, W105R, A378E, R93H, G623R, G703R, N219Y, R108C and G215S (Ledley et al. 1990, Jansen & Ledley 1990, Raff et al 1991, Ledley & Rosenblatt 1997, Qureshi et al. 1994, Acquaviva et al. 2001, Acquaviva et al. 2005, Berger et al. 2001, Worgan et al. 2006, Cavicchi et al. 2006).