Trimethylamine (TMA) is present in the diet (in fish) but primarily formed in vivo from the breakdown of choline. It is N-oxidised by FMO3 in the liver, the major isoform active towards TMA. Trimethylaminuria (TMAU; MIM:602079, fish-odour syndrome) is a human genetic disorder characterised by an impaired ability to convert the malodourous TMA to its odourless N-oxide. Patients emit a foul odour, which resembles that of rotting fish and can be a psychologically disabling condition (Messenger et al. 2013).