Immunofluorescence staining of wild-type PyMT mammary tumours (Figure 1F, Supplementary Figure S2) showed that the ColXV signal surrounds normal mammary ducts, where it resides juxtaposed and sometimes overlaps with the myoepithelial cell marker alpha smooth muscle actin (αSMA) (Supplementary Figure S2A–C). Here, ACTA1 is linked to breast cancer.