Fate-mapping studies in LysMCreTomato mice identified substantial numbers of Tomato+ myeloid cells with F4/80+ macrophage phenotype expressing α-SMA and collagen I in the unilateral ureteral obstructive model of renal fibrosis, providing direct evidence for the MMT process during the development of renal fibrosis. The gene discussed is ACTA1; the disease is renal fibrosis.