RB1 and lung cancer: Primary epithelial type II progenitor cells that serve as cells of origin for lung cancer were isolated from RB1/p16 deficient, p16 deficient, RB1 deficient and control RB1/p16-proficient lungs and serially passaged in culture using a modified 3T3 protocol to determine the role of RB1 and p16 in suppressing epithelial immortalization.