The maintenance/regulation of cellular levels of free fatty acids and fatty acyl-CoAs (the activated form of free fatty acids) is extremely important, as imbalances in lipid metabolism can have serious consequences for human health. Acyl-coenzyme A thioesterases (ACOTs) hydrolyse the thioester bond in medium- to long-chain fatty acyl-CoAs (of C12-C18 lengths) (MCFA-CoA, LCFA-CoA) to their free fatty acids (MCFA, LCFA) (Cohen 2013, Hunt et al. 2012, Kirkby et al. 2010). ACOTs that function in the mitochondrion are ACOT2 (Hunt et al. 2006), ACOT9 (Kirkby et al. 2010), THEM4 dimer (Zhuravleva et al. 2012, Zhao et al. 2012) and THEM5 dimer (Zhuravleva et al. 2012). THEM4 is also functional in the cytosol and at the plasma membrane (Cohen 2013).