Mitochondrial sarcosine dehydrogenase (SARDH) oxidatively demethylates sarcosine (SARC, aka methylglycine) to glycine (Gly) and formaldehyde (CH2O), an active 1-carbon unit (Eschenbrenner & Jorns 1999, ). SARDH requires one FAD as cofactor, which is reduced during the reaction. Defects in SARDH cause sarcosinemia (SARCOS; MIM:268900), a disorder characterised by an increased concentration of sarcosine in plasma and increased sarcosine excretion in urine. The clinical phenotypes of sarcosinemia are diverse, ranging from normal (most common) to ones associated with mental retardation, growth delay and muscular abnormalities (Bar-joseph et al. 2012).