We have compared the expression and distribution of three widely used markers for CAFs and myofibroblasts—α-smooth muscle actin (ASMA), platelet-derived growth factor receptor-α (PDGFRα), and fibroblast-specific protein-1 (FSP1)—in tumours from MMTV-PyMT mice of different ages and human breast cancers. This evidence concerns the gene ACTA1 and breast carcinoma.