p53 protein expression is knocked down by RNAi expression. The expression of p53 was examined by Western blot analysis of extracts from MCF-7, ZR-75-1, HME-CC, and ME16C cell line parents and the same cell lines stably transduced with p53-targeted RNAi vector or p53 mismatch (MM) RNAi vector. Treatment with doxorubicin (24 h, 1μM) induced p53 expression in all cell lines and transductants, but induced levels were markedly lower in the p53-RNAi cells.