The preimplantation embryos may have an increased risk of having mismatches due to the rates of cell proliferation and DNA replication. Elimination of mismatches in human gametes and embryos have not been investigated previously. In this study we developed a sensitive functional assay to investigate the repair or elimination of mismatches in both commercially available cell extracts and extracts obtained from preimplantation embryos. Heteroduplex molecules were constructed using synthetic

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... ucleotides. Efficiency of the repair of mismatches was semi-quantitatively analysed by exposure to nuclear/whole cell extracts (as little as 2.5µg) and extracts obtained from pooled mouse and human blastocysts to investigate the repair capacity in human embryos. A cell free in vitro assay was successfully developed to analyze the repair of mismatches using designed heteroduplex complexes. The assay was further optimised to analyse repair of mismatches in cell extracts obtained from oocytes and blastocysts using minute amounts of protein. Repair of mismatch efficiency was observed in both mouse and human blastocysts (2.5µg). The blastocysts were observed to have lower repair efficiency compared to commercially available nuclear and whole cell extracts. In conclusion, a sensitive, easy and fast in vitro technique was developed for the first time to detect the repair of mismatch efficiency in embryos. This study showed for the first time that the MMR was active in human embryos at the blastocyst stage.