PECAM1 and pulmonary fibrosis: To identify transcriptional changes in aged lung fibroblasts that impede fibrosis resolution, we carried out bulk RNA-Seq on fibroblasts sorted by FACS (EpCAM–, CD45–, CD31–, GFP+), isolated from the lungs of young (2 months) and aged (18 months) Col1a1-GFP transgenic mice during the initial phase of lung fibrosis resolution (30 days after intratracheal bleomycin instillation) (Figure 1A).