Abstract Mouse models of cancer have been instrumental in the development of our understanding of the processes that drive multistage carcinogenesis from the initiation stage to metastasis. Tumors induced in mice by exposure to carcinogenic chemicals replicate many of the biologic and genetic features of multistage cancer in human populations. Skin and lung tumors initiated by chemical mutagens generally have carcinogen-specific mutations in Hras or Kras, respectively, but the reasons for this selectivity are unknown. We have used mouse models of skin and lung cancer to address some of the unanswered questions in cancer biology related to the specific functions of Hras, Kras4A, and Kras4B in cancer development in vivo. The KRAS gene, encoding two splice isoforms (KRAS4A and KRAS4B), is mutated in many lethal human tumors and has become a major target for cancer drug discovery. We show that germline deletion of either the major Kras4B or the minor Kras4A isoform in mice confers complete resistance to development of Kras mutant lung cancers. Double heterozygotes that are able to express both isoforms, but in which splicing has been uncoupled, are also tumor resistant, suggesting that coordinated expression of both splice variants is necessary for cancer initiation in vivo. KRAS4A is enriched in human tumor cells with stem cell properties, and the two splice variants show different responses to inducers of hypoxia or ER stress. Our data help to rationalize previous conflicting information on metabolic changes in KRAS mutant tumors and suggest that KRAS4A and 4B may require separate targeting in cancer stem and progenitor cells. Citation Format: Allan Balmain. Tissue- and cell-specific functions of Hras, Kras4A, and Kras4B in skin and lung cancer [abstract]. In: Proceedings of the AACR Special Conference on Targeting RAS-Driven Cancers; 2018 Dec 9-12; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(5_Suppl):Abstract nr IA11.