Human mismatch repair protein 2 (MSH2) is involved in the initial recognition of mismatched nucleotides during the post replication mismatch repair process. Therefore, the loss of MSH2 function leads to the accumulation of replication errors, which in turn may be responsible for the multiple mutations required for multistage carcinogenesis. Mutations in mismatch repair genes have been linked to hereditary nonpolyposis colon cancer and to sporadic cancers that exhibit microsatellite instability. MSH2 is found in the nuclei of cells from a variety of tissues including thyroid, heart, smooth muscle and the germinal centres of lymphoid follicles. In ileum and colon, immunohistochemical staining has been reported in the crypts which are undergoing rapid renewal. They are responsible for the continuous production of differentiated cells which migrate over 2 to 4 days before being sloughed into the lumen. Human MSH2 is expressed in highly proliferative cells of the gut and any mutations in this gene could, therefore, be expected to expedite the progression of adenoma to carcinoma. This antibody, therefore, should prove of particular interest in the study of bowel tumours.