The Pulmonary adenoma resistance 2 (Par2) locus of the BALB/cByJ mouse, located within 0.5 cM of chromosome 18, is responsible for reducing the mean multiplicity of urethane-induced lung tumors relative to those in C57BL/ 6J, A/J and C3H/HeJ mice. Thus, BALB/B6-Par2 congenic strain genetically identical to BALB/cByJ except carrying C57BL/6J Par2 alleles develops seven times more tumors than BALB/cByJ. To gain clues for identification of Par2 candidate genes, we analysed lung tumorigenesis in BALB/cByJ2BALB.B6-Par2 chimeric animals. Of 100 tumors induced by urethane in 16 chimeras, 82 originated from BALB.B6-Par2 cells, indicating the Par2 phenotype to be cell-autonomous. In addition, the BALB.B6-Par2-and BALB/cByJ-derived tumors were similar in mean size, implying that the phenotype is primarily expressed during initiation rather than in the promotion stage of carcinogenesis. Given these results, we surveyed a comprehensive mouse genome database and physically mapped Par2 within a 2.3 Mbp segment containing three known genes, Polı, Mbd2 and Dcc. Among those, the Polı seemed to be the most reasonable Par2 candidate, since it encodes an extremely error-prone DNA polymerase preferentially incorporating G or T opposite template T in vitro, reminiscent of the Kras2 activation because of an A to G or T point mutation within codon 61 with which most urethane-induced lung tumors are initiated. Indeed, our sequencing of Polı cDNAs from BALB/cByJ, C57BL/6J, A/J and C3H/ HeJ lungs revealed 21 BALB/cByJ-specific single-nucleotide polymorphisms in the coding region accompanied by seven amino-acid substitutions and an elevated frequency of alternative splicing, while no polymorphisms associated with tumor susceptibility were found for either Mbd2 or Dcc. Notably, we obtained evidence that BALB/cByJ Par2 alleles may selectively decrease the frequency of Kras2-mutated tumors compared with C57BL/6J alleles. Consequently, the Polı is an intriguing Par2 candidate clearly deserving further evaluation.