MLH1 and colorectal carcinoma: This diagnostic strategy is complicated by the fact that MSI (or IHC lack of MLH1 and PMS2) is also seen in approximately 15% of sporadic CRC due to somatic biallelic methylation of the MLH1 promoter (Aaltonen et al. 1993; Boland et al. 1998; Cunningham et al. 2001).