Immunotherapies, such as CTLA-4 or PD-1 inhibitors, have revolutionized the treatment of metastatic melanoma patients. However, a key challenge to optimize the opportunity provided by these therapies is the dramatically varied responses among different patients, or even among different tumors in the same patient. For example, only a minority of melanoma patients will benefit from PD-1 inhibitors, whereas the remainder of patients have either incomplete or no response. Understanding the mechanism of these varied responses has the potential to improve patient care by identifying patients who will respond, and identifying novel drug targets that overcome resistance. We have identified changes in hundreds of genes (mutations) associated with the emergence of resistance to PD-1 inhibitor immunotherapy. Unfortunately, the sheer number of genetic changes and their interdependence makes it difficult to ascertain how each one particular gene impacts immunotherapy response. To solve this problem, we have developed a unique tool that recreates any resistance mutation in mice within several weeks. The mice can be used to evaluate the impact of mutations associated with immunotherapy resistance, as well as creating a platform for testing therapies that overcome resistance. In this proposal, we will use this tool to (i) elucidate the role of hundreds of mutations we have associated with immunotherapy resistance; and (ii) test a novel strategy using drugs in clinical trials that may overcome resistance. Our approach shows promise to transform the way we understand and treat resistance to immunotherapy.