The DNA mismatch repair (MMR) system has evolved in both highly conserved and highly complex manner like many other genome maintenance systems that are selected for during the evolution of life. MMR plays an essential role in genome maintenance by removing post-replication errors and by mediating a DNA damage response. Eukaryotic MMR is a complex system that requires the interactions of several MutS and MutL proteins for the initiation of the repair reaction. I have been focusing on functional studies of the MutSα complex, consisting of Msh2 and Msh6, during my doctoral studies. Despite intensive research efforts about MutSα complexes, many of the functions of MutSα in DNA repair and damage response still need to be elucidated. I generated three different MutSα-defective mouse models to elucidate the role of each of the two complex protein in the different functions of MMR with a special emphasis on their roles in tumor suppression. The specific aims of my study were: (1) To study the impact of Msh2 missense mutations on intestinal tumorigenesis in conditional Msh2 knockout mice. We generated and utilized the Msh2^loxP;Villin-cre mice (preclinical HNPCC model) to determine the response of tumors carrying an Msh2 knockout or a Msh2^GD “separation of function” mutation to chemotherapeutic treatment. (2) To determine the roles of MutSα in the repair of oxidative DNA damage and its significance for tumor suppression. To obtain more conclusive information regarding how defects in 8-oxoG repair is connected to cancer predisposition, we generated the MutSα^EA mutant mouse model carrying the Msh6^EA mutation in which the repair of 8-oxoG by MutSα is defective. High mutation frequencies caused by the accumulation of 8-oxoguanine and defects in cell cycle checkpoint signaling lead increased tumor susceptibility in Msh6^EA/EA mice. (3) To examine the roles of MutSα in MMR mediated DNA-Damage Signaling. The Msh6^SI/SI mutant mouse line has a defect in DNA damage signaling function, although it retains the normal MMR function. Studying this mutant mouse model will allow the further dissection of their MMR functions and provide information about the significance of the MMR-dependent DNA damage response function in tumorigenesis.