Since estrogens preferentially induce cyclin D1 to trigger breast cancer proliferation while p21 is transcriptionally regulated by ERRα to remove constraints in tumor progression [34], we evaluated the function of ERRα in the expression of these targets and in VDR protein expression to explore the potential regulatory mechanism of sensitivity to calcitriol mediated by ERRα. The gene discussed is CCND1; the disease is breast carcinoma.