SUMMARY Cell division and stress response differ among tissues and are perturbed in disease. This proposal focuses on a variation called the endocycle, during which cells periodically replicate their DNA without dividing, resulting in an increase in cell size and cellular DNA content (polyploidy). This alternative growth program occurs widely in nature, including many tissues in humans (e.g. liver, heart, blood, skin). Emerging evidence indicates that cells also switch to endocycles during wound healing, regeneration, and cancer. Despite their importance, much remains unknown about how endocycles are regulated and how they contribute to tissue growth and disease. The objectives of this proposal are to define the molecular and cellular mechanisms that regulate endocycles, their modified response to DNA damage, and their contribution to genome instability and cancer. Our previous studies showed that endocycling cells in the fruit fly do not undergo programmed cell death (apoptosis) in response to radiation or other treatments that damage DNA, a property that others have shown is shared by endocycling cells in mice. The repression of cell death acts through chromatin silencing of target genes of the p53 tumor suppressor. Our previous studies also showed that both fly and human cells can be induced to switch to endocycles, and then can switch back to mitotic divisions that are extremely error prone, resulting in daughter cells with abnormal DNA content (aneuploidy). It is known that human cancer cells are frequently aneuploid and that tumors contain giant polyploid cells. Together, these observations lead to the hypothesis that a transient switch to endocycles leads to cancer cell survival with a return to mitosis causing mutations that promote cancer progression. This proposal seeks to understand the molecular mechanisms that regulate endocycles, repress apoptosis, and contribute to tumor formation and metastasis. We will use the powerful tools in the fruit fly to achieve three specific aims: 1) To determine how the repression of apoptosis is linked to the endocycle program, 2) To define the mechanism by which CycA / CDK activates the Myb-MuvB to regulate alternative cell cycle programs, 3) To determine the contribution of transient endocycles to oncogenic growth and metastasis. The outcomes of this proposal will fill a major knowledge gap in understanding the regulation of the variant endocycle growth program and its contributions to development and cancer, ultimately leading to better regenerative medicine and cancer therapies.