p33(ING1) is a novel candidate tumor suppressor and its overexpression induces growth arrest or apoptosis in different cell lines. These functions of p33(ING1) depend largely on the activity of p53, and p53-dependent activation of the transcription from the p21/WAF1 promoter also requires p33(ING1). We examined the expression of ING1 mRNA in breast cancer cell lines and clinical breast cancer tissues, using quantitative RT-PCR and real time TaqMan technology. In breast cancer cell lines, ING1 mRNA was expressed at almost the same level. However, in a comparison between the cancer and matched normal tissues, a significant decrease in ING1 mRNA expression was found in 17 of 24 (70.8%) breast cancer tissues. We also examined the correlation between ING1 mRNA expression and p53 expression. There was a significant decrease of ING1 mRNA in nine of 15 tumors negative for p53 immunostaining, most of which were considered to have wild type p53. In these tumors, p53 may not function in case of a decreased expression of p33(ING1), and the lack of cell cycle regulation may correlate with the carcinogenesis and tumor progression.