Phenetic tree of a 21-mouse inbred strain of known susceptibility to lung carcinogenesis, obtained by the Fitch–Margoliash distance matrix method, with marker polymorphisms mapped in the YAC contig containing the Pas1 gene. The tree represents the estimated relationships among the strains for that short chromosomal region (∼1.5Mb), and it therefore provides an estimation of the phylogenetic origin of thePas1 ^s allele. The gray box contains the putativePas1 ^s mice, which include the A/J, SWR/J, and BALB/c strains proved by genetic linkage to carry the Pas1 ^sallele. Also, the putative Pas1 ^r strains include the C57BL/6J, C3H/HeJ, and M. spretus mice, established by genetic linkage studies as Pas1 ^r strains.