Evaluation of spontaneous Cre recombination in Tg(BK5-CreERT)S;R26R mice. a: E14.5 embryos (left panel, R26R/+ (R26R) control; right panel, Tg(BK5-CreERT)S/+;R26R/+ abbreviated as K5-R26R) were stained for β-galactosidase activity, which appeared in whisker and hair follicles of the K5-R26R embryo (arrows). b: Dissected stomach from newborn mice (P1) after whole mount X-gal staining revealed the presence of intensive blue-stained areas in the esophagus (E) and forestomach (FS; arrows) of K5-R26R mice. c & d: Dissected thymus from R26R or K5-R26R mice underwent whole mount X-gal staining and histological section and this revealed scattering blue foci (arrows) in the K5-R26R thymic medulla (M) at P7 (d) but not at P1 (c). e: The urogenital system of R26R or K5-R26R females at P7 was dissected and stained for β-galactiosidase activity. Histological sections of corresponding X-gal stained organs of a K5-R26R female are shown in the right panels. Arrows indicate positively stained epithelium; B, bladder; U, uterus; V, vagina; SV, sinus vagina; MV, Müllerian vagina; f & g: Histological sections of X-gal stained skin and trachea from adult offspring (R26R and R26R;Tg(BK5-CreERT)S) obtained by crossing a R26R/R26R male with a Tg(BK5-CreERT)S female. The β-galactosidase activity showed tissue/cell-specific expression in the epidermal, hair follicle and sebaceous cells of the R26R;Tg(BK5-CreERT)S skin (arrows in f) and in the basal cells of R26R;Tg(BK5-CreERT)S trachea (arrows in g). Scale bars, 100 μm.