Breast cancer is the most prevalent cancer in women, and most cases are believed to have a sporadic, rather than heritable basis. Therefore, a major challenge in cancer research is to determine the underlying genomic alterations leading to carcinogenesis and malignancy, and then use this information for personalized therapies. Genomic studies of human cancers that aim to identify causative mutations are complicated by the prevalence of passenger mutations, genetic heterogeneity, and the diversity of breast cancer etiologies and tumor subtypes. Mouse cancer models are powerful for untangling the genomic basis of cancers because genetic and phenotypic variation can be eliminated or controlled. To identify genes contributing to mammary tumorigenesis, we exploited the C3H-Mcm4Chaos3/Chaos3 (“Chaos3”) mouse model that, by virtue of bearing a defective DNA replicative helicase subunit that causes elevated genomic instability (GIN), sustains somatic alterations ultimately causing mammary adenocarcinomas. Genomic analysis of Chaos3 mammary tumors revealed recurrent copy number alterations (CNAs) of specific genomic regions, most notably deletion of the Neurofibromin 1 (Nf1) tumor suppressor gene in all cases. NF1, a negative regulator of RAS, is traditionally recognized for its role in driving the development of neurofibromas in the context of the human disease Neurofibromitosis but not breast cancer. We observed elevated RAS activation and increased sensitivity of both Chaos3 and human Nf1-mutated breast cancer lines to MAPK and/or PI3K/AKT pathway inhibitors. We also found striking overlap between Chaos3 CNAs and human breast cancer CNA data curated in public genomic databases, including Nf1 deletion. Together, our results indicate that spontaneous NF1 loss can drive breast cancer and suggests a potential therapeutic strategy in that subset of patients. reference x sample