A, Whole-mount β-galactosidase assay at E9.5 revealed loxP recombination in the pharyngeal area, the epicardium (arrow) and somites. B, Staining in all four limb buds became very prominent at E12.5. C, lacZ-positive cells were detected in the epicardium (arrow) and sparsely in the myocardium (triangle). D, an image of the E15.5 heart showed epicardial staining. Additional expression in the seminal vesicle (arrows in E) was observed. Cre-mediated recombination was also detected in progenitor cells for the vibrissae (arrows in F).