Abstract Historically metastatic melanoma has been associated with poor prognosis. The advent of immune checkpoint inhibitors targeting CTLA-4 and PD-1 have greatly improved patient survival in metastatic melanoma, but a significant proportion of patients fail to respond to therapy. To explore the effect of genetic background on immune checkpoint inhibitor efficacy, we have developed a tumor immunotherapy model using genetically heterogeneous Diversity Outbred (DO) mice and the C57BL/6 syngeneic B16F0 melanoma model. To implement genetic diversity, DO mice were crossed with C57BL/6 to generate (C57BL/6xDO)F1 mice. Untreated (C57BL/6xDO)F1 mice (n=34) reliably develop B16F0 tumors after subcutaneous inoculation, with some variation in tumor latency. To test the role of genetics in response to ICI, (C57BL/6xDO)F1 mice (n=142) were treated with combined anti-PD1/anti-CTLA-4 on days 3, 6, and 10 after inoculation with 2×105 B16F0 cells. Mice receiving therapy show wide variation in tumor development of up to 65 days (mean 20.86 +/− 11.04, CV 52.94%), with 19 never developing tumor by day 88. Re-challenge of tumor free mice confirmed adaptive response to tumor. Preliminary Genome Wide Association shows suggestive associations on chromosomes 8, 12, and 13. The locus on chromosome 13 contains a family of related genes with known function in autoimmune conditions and expression in several immune cell types. QTL effects show the NZO genotype is a positive driver at this locus and C57BL/6 is a negative driver, reflecting known ICI resistance in the C57BL/6 strain. These results demonstrate host genetic background significantly contributes to response to ICI and identifies genomic loci associating with primary resistance. NIH R37 CA220482.