Lung cancer is the leading cause of cancer death among both men and women, accounting for more than 28% of all cancer deaths. In fact, more people die of lung cancer than of colon, breast, and prostate cancers combined. Although lung cancer is largely induced by smoking, there is strong evidence for genetic susceptibility and gene-environment interactions in the development of lung cancer. Inbred mouse models offer an effective means of identifying candidate lung cancer susceptibility loci since genetic heterogeneity and enormous variation in exposure levels to environmental agents make it difficult to identify lung cancer susceptibility loci in humans. Papg-1 (pulmonary adenoma progression 1) was previously mapped to a region on mouse chromosome 4. This locus contains a candidate gene, Cdkn2a also referred to as Ink4a/Arf, which dually encodes two established tumor suppressors p16(INK4a) and ARF. Cdkn2a became a primary candidate for Papg-1 for two reasons: (1) two haplotypes of mouse Cdkn2a were found to segregate with differential genetic susceptibility to lung tumor progression in mice; and (2) in vitro studies showed that the p16(INK4a) allele from the BALB/cJ mouse had a significantly decreased ability to bind and inhibit CDK6 and to suppress cell growth when compared with the p16(INK4a) allele from the A/J mouse. Here, we report that mice with a heterozygous deficiency for the A/J Cdkn2a allele were significantly more susceptible to lung tumor progression than mice with a heterozygous deficiency for a BALB/cJ Cdkn2a allele, when compared to their respective wild type mice. These results offer strong evidence that naturally occurring variation of p16(INK4a) influences susceptibility to enhance lung tumor progression making it a strong candidate for the lung tumor progression locus, Papg-1.