One of the features of cancer cells is their genomic instability. The proper function of DNA repair system is crucial to remove DNA damage and maintain genomic integrity in living cells. DNA Mismatch Repair System (MMR) is a key post replicative system that corrects potentially mutagenic DNA mispairs and insertion/deletion loops. MMR defects cause predisposition to certain types of cancer, including colon, endometrial, and ovarian cancer. In addition, the loss of MMR function has been associated with resistance to several DNA damaging anti-cancer agents. Heat Shock Proteins (HSPs) belong to a superfamily sharing a protection function against cellular stress, thus contributing to cellular protein homeostasis. They play an important role assisting the correct folding of nascent and stress-accumulated misfolded proteins, and preventing their aggregation. Hsp27 (HSPB1) and Hsp27 (HSPA1A) are overexpressed in many tumor cells and are known to have anti-apoptotic properties. In addition, they have been implicated in the development of resistance to anticancer drugs. They may contribute to cell survival inhibiting apoptosis or increasing DNA repair. Many studies have reported a link between HSPs and DNA repair pathways, wich has become a special topic of interest in oncology research. However, the relationship between Hsp27 and Hsp72 with the MMR pathway remains poorly explored. We recently reported that MSH2 and MLH1 (components of MMR) are client proteins of Hsp27 and Hsp72, and that cisplatin exposure increased the nuclear assciations between HSPs and MMR proteins in colon cancer cells. The same result was observed for Gili cells, where the interaction between Hsp27 and MSH2 increased after temozolomide treatment. However, functional MMR studies are needed to elucidate wether HSPs are stimulating the repair of drug-induced DNA damage or whether these interactions are only a result of their chaperone functions. The aim of this project is to determine whether Hsp27 and Hsp72 particpate in DNA mismatch repair system. In order to achieve this goal, it is necessary to perform a MMR activity assay. The in vitro MMR assay has been effectively used to assess MMR proficiency in various tumot cells and to identify the majority of the protein components required for MMR. This powerful method is very complex and the procedure involves: 1) preparation of the nuclear extracts from tumor cell lines, 2) preparation of DNA heteroduplex substrate and 3) the in vitro MMR assay. The discovery of the role of HSPs in the MMR system will contribute to design potential targets for cancer therapy in order prevent the chemoresistance.