CDKN1A and chronic obstructive pulmonary disease: Chronic LPS inhalation (repeated exposure of E. coli LPS aerosol for 10 weeks) of normal and chronic obstructive pulmonary disease mice increased the expression of p21/WAF-1, γ-H2AX, and SA-β-Gal+ cells in bronchial epithelial cells [90] or lung tissues [91].