Mammalian meiosis has become a major focus for research in recent years, in part due to the increased availability of mouse mutants affecting meiotic recombination. Such studies have highlighted the importance of one group of proteins in particular, the mismatch repair protein family, which were hitherto associated only with repair of DNA during mitosis. Three of the four MutL homologs (MLH1, MLH3 and PMS2) that belong to this family are essential regulators of mammalian meiosis, with MLH 1 and MLH3 being the only proteins that are localized specifically to sites of reciprocal recombination. In the absence of MLH1 and MLH3, chiasma maintenance is disrupted, resulting in the premature separation of homologous chromosomes prior to the first meiotic division. These observations implicate MLH1 and MLH3 in the establishment and maturation of crossover structures in mid-prophase I. The role of PMS2 is less certain, but it too is essential for meiotic progression. Moreover, the functional localization of MLH1 is dependent on the prior loading of MLH3, suggesting an inter-dependence between family members. The aim of the current proposal is to examine these relationships and to explore the individual MutL homolog functions during meiosis in both males and females. Aim 1 will focus on the role of MutL homologs in prophase I progression. Using single/double MutL-deficient mouse strains (for Mlh1, Pros2 and Mlh3), we will examine the role of each protein in chromosome synapsis, crossing over and chiasma maintenance in both males and females, and will examine the localization of each MutL homolog in the different mutants. Leading on from this, in aim 2, we will examine the key protein-protein interactions that involve each MutL homolog as a means of identifying unique functions for each of these family members. In Aim 3, we will define a novel system for monitoring meiotic progression in situ by generating mice harboring a fluorescent COR1/SCP3 protein fusion transgene. COR1 is a synaptonemal complex component which forms a backbone along homologous chromosomes in prophase I, allowing us to monitor their interactions in real-time. When crossed with different MutL homolog deficient mice, therefore, we will be able to examine the effects of these mutations on chromosome interactions and nuclear dynamics. Taken together these studies will provide a complete and thorough comparison of the meiotic roles for individual MutL homotogs and will provide a novel and exciting method for tracking normal/abnormal chromosomal events during prophase I.