CDKN2A and chronic obstructive pulmonary disease: CS exposure increases the levels of markers of prosenescence (i.e., p16, p21, p53) and DNA damage (i.e., γH2AX phosphorylation) in human lung epithelial cells and fibroblasts in vitro, acute/chronic CS-exposed mouse lungs in vivo, and in lungs of patients with COPD [22–26].