PROJECT SUMMARY Somatic mutations in the Neurofibromatosis type 1 (NF1) tumor suppressor gene are reported in 15-25% of melanomas and are implicated in activated mitogen-activated protein kinase (MAPK) signaling and tumor growth. The clinical and biological relevance of NF1 mutations is poorly understood. Evidence suggests that NF1-mutant (NF1-MT) melanoma is associated with a higher tumor mutation burden (TMB). However, NF1 mutations are not associated with better response of melanoma to immunotherapy. Studies have also suggested that NF1-MT melanomas are sensitive to MEK or ERK inhibitors, but a recent report showed that NF1 mutations do not predict sensitivity to these drugs. To date no actionable targets have been identified and validated in NF1-MT melanoma, and no subtype-specific therapy that exploits its biology has been developed. Our integrated genomic, transcriptomic, and proteomic analyses suggest that increased cell cycle progression is a hallmark of NF1- mutant melanomas, which might be exploited as a novel treatment strategy. Hence, we hypothesize that NF1- mutant melanomas are biologically distinct from NF1-wild type tumors, and possess a hyper-proliferative phenotype, that cannot be explained only by MAPK activation and characterized by increased cell cycle progression, which renders them vulnerable to pharmacological inhibition of cell cycle progression and cell proliferation, such as agents targeting CDK4/6 or CDC20, alone or in combination with MEK inhibition, or inhibition of Shp2, a phosphatase that contributes to MAPK pathway activation independently of mutant BRAF or NRAS. In this project, we will use melanoma patient tissues, cell lines, and patient-derived tumor models in mice to elucidate the biological and clinical impact of NF1 mutations in melanoma, and to test novel strategies to treat this distinct melanoma molecular subtype by targeting cell cycle progression. Our findings will provide the basis for further development of innovative, subtype-specific strategies for treatment of NF1-mutant tumors.