CYP24A1 and breast cancer: Although BRCA1-silenced MCF7 and MDA-MB-231 breast cancer cells are still responsive to vitamin D3, as reflected by Cyp24A1 mRNA induction, the G1/G0 checkpoint and p21waf1 induction were impaired despite similar expression leveles of vitamin D receptor (VDR) and its dimerization partner protein, RXR.