CCND1 and breast carcinoma: It was found that siRNAs targeting Cyclin D1 not only restored tamoxifen sensitivity in MCF7-Re and T47D-Re cells (Fig. 1b and S1g), but also resulted in cell cycle arrest at G1 phase (Fig. S1h, i), indicating that these tamoxifen-resistant breast cancer cells are still dependent on Cyclin D1 for cell cycle progression and upregulated Cyclin D1 is responsible for their tamoxifen resistance.