Most sporadic MSI-H cases are associated with somatic MMR deficiency caused by the CpG Island Methylator phenotype (CIMP), in which the MLH1 promoter shows biallelic somatic methylation with B-Raf Proto-Oncogene, Serine/Threonine Kinase (BRAF) mutations usually present in the background (almost never seen in LS) [41]. The gene discussed is MLH1; the disease is Leigh syndrome.