The myoepithelial layer in DCIS is murine in origin. Ducts containing DCIS 3 weeks after intraductal injection of 4G-shp53-PI3K tumor cells were costained for p63 (red), GFP (green) and DNA (DAPI, blue) (n = 4). The p63-positive cells do not stain for GFP, indicating that they are murine in origin. Red arrow, normal murine duct; green arrow, DCIS. Scale bars 50 μm.