"The mismatch repair (MMR) system has evolved to correct errors of DNA replication and prevent recombination between non-homologous sequences. Correspondingly, MMR malfunction leads to increased mutation rates and illegitimate recombination, and individuals with inherited MMR gene mutations are predisposed to cancer of the colon and other organs. However, MMR has recently been implicated in processes ranging from DNA damage signaling and drug sensitivity to antibody maturation, some of which contradict and even subvert the classical role of MMR as a guardian of genomic integrity. We suspect that the latter processes are linked to a new, non-canonical MMR (ncMMR) pathway that can be activated outside of the S- and G2 phases of the cell cycle by a variety of lesions and structures. ncMMR lacks strand directionality, involves long stretches of DNA degradation, and our preliminary in vitro evidence suggests that resynthesis of these repair tracts can be mediated not only by high-fidelity, replicative polymerases, but also by error-prone enzymes. In this scenario, ncMMR would actually contribute to mutagenesis. I plan to deploy proteomic, genomic and imaging technologies to identify the components of the ncMMR “mutasome” and to reconstitute the system from purified recombinant components. Furthermore, I wish to study the “action radius” of MMR proteins by characterizing their interactome and analyze its dependence on endogenous states (e.g. cell cycle stages) and exogenous stimuli (e.g. drug treatments) in human and chicken (DT40) cells, and Xenopus laevis egg extracts. I intend to exploit a new system of inducible protein replacement that was recently developed in my laboratory, to stably express MMR, replication, repair and recombination proteins (both wild type and variants). This program should increase our understanding of the pivotal role of MMR in DNA metabolism and its involvement in human disease and cancer."