KRT5 and ductal breast carcinoma in situ: Based on previous studies by our group15–18 and cytokeratin 5 stainings for myoepithelial cells, 4T1 tumor cells remained inside the mammary ducts surrounded by an intact epithelial barrier until 7 days post inoculation (p.i.), characteristic for the ductal carcinoma in situ (DCIS) stage (Fig. 1a).