CDKN2A and chronic obstructive pulmonary disease: To validate the clinical relevance of these findings, we stained lung tissue sections derived from Control, IPF, and COPD patients for Podoplanin (PDPN), a structural marker for alveolar epithelium and collagen triple helix repeat containing 1 (CTHRC1), a recently described marker for fibroblasts [32] in combination with the senescence marker CDKN2A/P16.