CYP27B1 and type 1 diabetes mellitus: In the kidney, 1-α-hydroxylase, encoded by CYP27B1 converts 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D, directly affecting the circulating levels of active vitamin D. Polymorphisms within CYP27B1 have previously been associated with the development of type 1 diabetes [5].