Progesterone receptor (PR)/bromodeoxyuridine expression and co-localization in keratin 6ab (K6ab)-null mammary glands. Mammary gland sections from wild-type (a) or K6ab-null (b) transplanted tissue after 10 weeks of outgrowth were treated for 2 days with estrogen and progesterone and stained for PR (red) and BrdUrd (green) by indirect immunofluorescence. The inset images show increased magnification of double-positive cells. White arrows indicate proliferating myoepithelial cells. Scale bar, 50 μm. (c) Cells positive for each marker, or those that co-localized (yellow arrowheads), were quantified and expressed as a percentage of total luminal epithelial cells. Error bars represent the SEM, and asterisks indicate statistical significance (p < 0.05).