The microsatellite instability (MSI), which is characterised by the absence of protein expression encoded by the corresponding MMR genes (hMLH1, hMSH2, hMSH6 or PMS2;Thibodeau et al, 1993, 1998; Popat et al, 2005), is observed in nearly all patients with CRC due to hereditary non-polyposis colon cancer (HNPCC) (Aaltonen et al, 1993, 1994; Thibodeau et al, 1993) and in 15–20% of patients with sporadic CRC (Aaltonen et al, 1993). The gene discussed is MSH6; the disease is hereditary nonpolyposis colon cancer.