side-chain-hydroxylated D3derivatives was explored. We found that the 24R and 26R metabolites were more effectively hydroxylated at C1 by CYP27B1 compared to the corresponding S diastereomers. However, CYP27B1 showed almost no activity towards either of the diastereomers of the 23-hydroxylated derivative. This is the first report to show that CYP27B1 metabolizes 26-hydroxylated D3, converting 25,26D3