Phosphate-buffered saline (PBS) was prepared from PBS tablets (OXOID PBS; Unipath Ltd., Basingstoke, Hampshire, UK). The following were used: mouse monoclonal antibody anti-p63 (Santa Cruz Biotechnology, Santa Cruz, CA), anti-BCRP (also known as anti-ABCG2) antibody (BXP21) (Calbiochem, San Diego, CA), anti-connexin 43 (Sigma-Aldrich, St. Louis, MO), nuclear antigen Ki67 (Immunotech, Marseille, France), bromodeoxyuridine (BrdU) and anti- BrdU (Invitrogen-Molecular Probes, Eugene, OR), fluorescein Alexa-488 conjugated goat anti-mouse and goat anti-rabbit secondary antibodies (Invitrogen-Molecular Probes), fluorescein Cy3 conjugated goat anti-mouse and goat anti-rabbit secondary antibodies (Jackson ImmunoResearch Laboratories, Inc., West Grove, PA), reduced-serum medium (Opti-MEM I; Invitrogen-Gibco, Grand Island, NY), OCT compound (Tissue-Tek; Sakura Finetek, Torrance, CA), and slides (Superfrostplus; Menzel-Gleser, Braunschweig, Germany). Fetal calf serum (FCS) and goat serum were both obtained from Invitrogen-Gibco. Antifade medium (Citifluor) was purchased from Agar Scientific (Stansted, UK).