MMTV/MDM2DD mice showed delayed mammary involution. (A-F) Histological patterns on involution 1 day and 3 days mammary glands from FVB/n (A, D), MMTV/MDM2DD (B, E) and MMTV/MDM2 (C, F) mice. (G, H) TUNEL analysis on involution 3 days mammary glands from FVB/n (E) and MMTV/MDM2DD (G) mice. Arrows indicated the apoptotic cells. (I, J) Immunohistochemical analysis of p53 expression on mammary glands from FVB/n (I) and MMTV/MDM2DD (J) mice. (K) Apoptotic indices of FVB/n and MDM2DD mice at involution 3 days. (L) Immunoblot analysis of p53 expression on mammary glands from FVB/n and MMTV/MDM2DD mice. (M) Quantitative RT-PCR analysis of Tweak transcript expression in mammary glands from MMTV/MDM2DD and FVB/n mice. (O) Cell number counting on MECs from MMTV/MDM2DD and FVB/n mammary glands with etoposide (25ug/ml) treatment for 3 days. Re-expressing of p53 was achieved by p53-containing lentivirus infection for 2 days.