ER beta has been shown to inhibit human breast cancer cell proliferation by repressing transcription of the c-myc, cyclin D1 and cyclin A genes and increasing the expression of the cyclin-dependent kinase inhibitors p21Waf1/Cip1 and p27Kip1, leading to cell cycle arrest in the G2 phase [5,40,61]. Here, CCND1 is linked to breast cancer.