CYP27B1 and breast cancer: Both of these trials are conducting serial breast tissue sampling before and after the 1-year intervention to evaluate target tissue effects of vitamin D. For example, differential mRNA and protein expression of 1α-hydroxylase (CYP27B1, the activating enzyme) and 24-hydroxylase (CYP24A1, the deactivating enzyme) were demonstrated between breast cancer and benign breast tissue [28].