Flow cytometric analysis suggested COPD SAECs were mainly in G1 arrest (Fig. 3B), with elevated levels of the cell cycle checkpoint inhibitors (p16INK4a, p21Waf1, and p27Kip1) (Fig. 3C–E), and reduced expression of sirtuin-1 and CDK4 (Fig. 3F, G) compared with cells from age-matched nonsmokers. The gene discussed is CDK4; the disease is chronic obstructive pulmonary disease.