CDKN1A and idiopathic pulmonary fibrosis: Other markers of senescence, notably expression of the DNA damage marker γH2A.X and the associated cell cycle inhibitor p21Waf1/Cip1, were both significantly increased at the protein level in fixed IPF lung fibroblasts compared to normal human lung fibroblast (NHLF) cells by fluorescent high content imaging analysis (Figure 2A, 2B).