MSH2 and MLH1 are proteins involved in DNA reparation. They are mutated in
some forms of colon cancer, i.e. hereditary non-polyposis. carcinomas and a
subset of sporadic carcinomas. We have studied the expression of MSH2 and
MLH1 in a retrospective series of 225 colorectal carcinomas by immunohistoc
hemistry. The results were compared to molecular tests of microsatellite in
stability using amplification by PCR of BAT-25 and BAT-26 repeated sequence
s and to histoclinical data. Positivity of both proteins was never associat
ed with MSI phenotype. MSH2 and/or MLH1 negative tumors were frequently tum
ors of the proximal colon; in this subpopulation or proximal tumors, MSH2 a
nd/or MLH1 negativity was associated with a longer disease-free survival.