CDKN1B and chronic obstructive pulmonary disease: 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.