In order to identify the fibroblast origin after myocardial infarction, Kanicicak and collaborators performed a lineage tracing study using inducible Cre under the control of transcription factor 21 (Tcf21, marking resident fibroblasts) or myosin heavy chain 11 (Myh11, marking smooth muscle cells) promoter with a lacZ reporter. The gene discussed is TCF21; the disease is myocardial infarction.