Aberrant promoter hypermethylation is a hallmark of CRC that has a pivotal role in defining tumour subtypes; somatic methylation of the MLH1 promoter causes the majority of sporadic CRC tumours with microsatellite instability (MSI), and widespread CpG island methylation defines the CpG island methylator phenotype (CIMP). The gene discussed is MLH1; the disease is colorectal carcinoma.