The aims of this proposal are to define how human MAD2 (hsMAD2) functions as a mitotic checkpoint gene and to determine if disruption of hsMAD2 contributes to the generation of neoplasia in mouse models. hsMAD2 is the human homolog of a mitotic checkpoint gene first identified in budding yeast (scMAD2). The investigator proposes to test the hypothesis is that the biochemical mechanism underlying human MAD2 checkpoint function is its ability to inhibit the APC through its interactions with Cdc16 and Cdc27. In addition, MAD2 will be disrupted in both ES cells and mice to determine the contribution made by MAD2 to mitotic checkpoint control in mammalian cells.