Analysis of spontaneous Cre recombination in Tg(BK5-CreERT)S;R26R mammary glands. a-c: Whole mount X-gal staining of mammary fat pads; Arrows indicate ductal and alveolar structures that show X-gal positive staining in the K5-R26R mammary glands of virgin (b) and lactation (c) stage mice; this staining is not present in R26R mice (a). d-f: Histological sections of the corresponding X-gal stained mammary glands shown in a-c. Arrows indicated X-gal stained basal and luminal epithelial cells. a-c, cropped original magnification ×8; d-f original magnification ×200.