Melanoma prevention and treatment strategies are transitioning to an individualized and mechanism-based approach, based on genetics. However inherited variation between individuals can manifest at any stage of neoplastic progression, and possibly the interaction of many genes is involved. Except for family history, the presence of above average numbers of naevi is the strongest melanoma risk factor. Naevus count is mostly genetically determined, although genes discovered to date explain only a small proportion of the heritability. As Dysplastic Naevus Syndrome (DNS) is at the upper extreme of naevus count, a powerful strategy to better understand melanoma progression is to look for genes that modify this phenotype. To do this we have combined our mouse model of DNS (Cdk4R24C::Tyr-NRAS) with the Collaborative Cross (CC), a large resource of inbred mouse strains. The CC facilitates not only rapid mapping of genes, but also provides a step-by-step way to move from quantitative trait locus, to gene, to pathway, to biology. We have mapped several melanoma-related phenotypes to very small chromosomal regions, containing only a very small number of genes. The identification of these genes, and the molecular pathways through which they act, will lead to a fuller understanding of naevus and melanoma development not only in DNS patients (who are at the extreme), but also in the general population. Especially attractive as potential therapeutic targets are genes that slow the development of lesions, as they are dominant over the oncogenic mutations known to generate melanoma in both our mice, and humans (CDK4R24C and NRASQ61K).