VDR and breast cancer: Although 1,25(OH)2D2 and 1,25(OH)2D3 were shown in separate studies to have equal affinities for VDR in chick intestine, rat intestine, porcine kidney, human breast cancer cells and HeLa cells [27–29], 1,25(OH)2D3 was able to up-regulate rat intestinal VDR more than 1,25(OH)2D2 did [30], and differences in the effects of 1α-cholecalciferol and 1α-ergocalciferol were shown in rat intestinal calcium absorption, and osteoclast numbers and activity [31,32].