MRC1 and Leigh syndrome: Exceptionally, microsatellite stable (MSS) tumours with no BRAF mutation and no methylation of MMR genes may be LS associated (Julie et al, 2008), and caused by the less penetrant MSH6 gene that is mutated in approximately 5–10% of LS tumours, whereas mutations of MLH1 and MSH2 account for approximately 90–95% of LS tumours (de la Chapelle, 2004).