Inbred strains of mouse show various susceptibilities to spontaneous and chemical-induced lung tumorigenesis. Genetic analyses have revealed that lung tumor susceptibilities of inbred mouse strains are governed by quantitative trait loci (QLTs) located on multiple chromosomes. A major lung tumor resistance QLT, designated pulmonary adenoma resistance 2 (Par2), was mapped to the mouse chromosome 18 independently by several groups and accounted for up to 60% phenotype variance between susceptible A/J and more resistant BALB/c strains. The authors recently conducted studies to positionally clone the Par2 gene. This review summarizes the effort and progress towards the identification of Par2 candidates.