Moreover, we previously reported that the Acta2-Cre-ERT2; tdTomatoflox lineage-tracing tool allows genetic labeling of ACTA2+ myofibroblasts as they accumulate during the buildup of lung fibrosis when tamoxifen is introduced between days 5 and 14 after bleomycin injury8. Here, ACTA2 is linked to pulmonary fibrosis.