Objective Investigation of the effects of sodium selenite on genetic instability of human tumor cells via its role in alteration of microsatellite sequence(MS) of RER + (replication error) colon cancer cell line. Methods RER + and RER - human colon cancer cell lines, RKO or SW480, were used as hosts for lipofection with pcmv car in which a foreign(CA) 14 repeat was inserted in the coding sequence of LacZ reporter gene, resulting in misreading LacZ frame. Any mutation which makes the base number of (CA) 14 tract to be 3 fold will resume the normal reading frame of the reporter gene, and thus lead to expression and production of bioactive β galactosidase. To test the effect of selenite on MI(microsatellite instability) of tumor cells, a series of concentrations of selenite were administered in cell culture in vitro. Variable expression of bioactive β galactosidase of transfectant cells resulted from selenite administration was measured by A reading at λ410 after X gal staining; Results Mutations of the exogenous(CA) 14 developed and maintained in pcmv car transfectant RKO cells but not in SW480 cells. It was found that blue cell frequency of RKO transfectant cells was markedly reduced after incubation of cells with 5 μmol/L of selenite for 5 days, at which concentration it was not toxic to cell growth. However, selenite at lower concentration of 1μmol/L didn′t exhibit suppression of blue cell rate until cell′s exposure to it for a longer period up to 5 weeks or more. Conclusion Our data showed that selenite displayed inhibitory effect on MI of human cancer cells and thus demonstrated its beneficial role in stabilization of human genomic DNA. Objective Investigation of the effects of sodium selenite on genetic instability of human tumor cells via its role in alteration of microsatellite sequence (MS) of RER + (replication error) colon cancer cell line. Methods RER + and RER - human colon cancer cell lines, RKO or SW480, were used as hosts for lipofection with pcmv car in which a foreign(CA) 14 repeat was inserted in the coding sequence of LacZ reporter gene, resulting in misreading LacZ frame. Any mutation which makes the base number of (CA) 14 tract to be 3 fold will resume the normal reading frame of the reporter gene, and that lead to expression and production of bioactive β galactosidase. To test the effect of selenite on MI(microsatellite instability) of tumor cells, a series of concentrated of Selenite were administered in cell culture in vitro. Variable expression of bioactive β galactosidase of transfectant cells resulted from selenite administration was measured by A reading at λ410 after X Gal staining; Results Mutations of the exogenous(CA) 14 developed and maintained in pcmv car transfectant RKO cells but not in SW480 cells. It was found that blue cell frequency of RKO transfectant cells was markedly reduced after incubation of cells with 5 μmol/L Of selenite for 5 days, at which concentration it was not toxic to cell growth. However, selenite at lower concentration of 1μmol/L didn’t exhibit suppression of blue cell rate until cell′s exposure to it for a longer period up to 5 Weeks or more. Conclusion Our data showed that selenite displayed inhibitory effect on MI of human cancer cells and thus demonstrated its beneficial role in stabilization of human genomic DNA.