ACTA1 and fibrosis: Furthermore, whereas apoptotic S100A4+ and α-SMA+ fibroblasts were detected in the lungs of bleomycin-instilled Dermo1-Cre;Fas+/+ and Col1-CreERT2;Fas+/+ mice at 3 and 4.5 weeks, they were not detected in Dermo1-Cre;Fas–/– and Col1-CreERT2;Fas–/– mice, suggesting that Fas signaling plays an essential role in fibroblast apoptosis during homeostatic fibrosis resolution.