Since loss of heterozygosity (LOH) is one of the mechanisms for gene inactivation, and previous studies have detected LOH at the hMLH1 locus in HNPCC tumours (Hemminki et al, 1994; Wheeler et al, 2000), it is important to know whether LOH mediates the inactivation of hMLH1 in sporadic cancer. Here, MLH1 is linked to hereditary nonpolyposis colon cancer.