CDKN1B and idiopathic pulmonary fibrosis: To confirm the status of the lung fibroblasts before we proceed with the next experiments, we performed co-staining of SA-β-gal and the DNA damage marker γ-H2Ax, a SA-β-Gal staining and a RT-PCR for the senescence markers CDKN1A, CDKN1B, CDKN2A and WNT16. The elevated expression of p21 (CDKN1A), an important marker of cellular senescence, was observed in senescent when compared to proliferating lung fibroblast in both normal and IPF cells (Fig. 1b).