Among the breast cancer cell lines investigated, MCF-7 cells exhibited the strongest inhibitory effect on cell proliferation after cyclin B1 siRNA treatment, followed by MDA-MB-231, SK-BR-3 and BT-474 cells (Fig. 2), which possibly correlates with the cyclin B1 level in exponential growing status of each cell line (data not shown). The gene discussed is CCNB1; the disease is breast cancer.