CD24 and Sca-1 expression distinguishes basal, luminal ER- and luminal ER+ mammary epithelial cells. A) Section through a branching duct in a mouse mammary gland stained for expression of keratin 14 in the basal myoepithelial layer (K14; red) and keratin 18 in the luminal epithelial layer (K18; green) and counterstained with DAPI (blue) to distinguish nuclei. Bar = 60 μm. B) Section through a mouse mammary duct stained for K14 (red) and ER (green) and counterstained with DAPI. Only a subset of luminal cell nuclei show ER staining (examples indicated by arrows). Bar = 25 μm. C) Flow cytometry plots of freshly isolated mouse mammary epithelial cells stained with control IgG (left) or anti-CD24 and anti-Sca-1 (right) antibodies. The regions corresponding to the CD24+/Low Sca-1- (red), CD24+/High Sca-1- (green) and CD24+/High Sca-1+ (blue) populations are indicated.