This 25(OH)D3 is carried in
the blood to enter multiple cell types where a 25(OH)D3-1a-hydroxylase (encoded by the CYP27B1 gene) adds another hydroxyl
group to the 1 position to form the active hormone 1,25(OH)2D3,
which will be referred to as Vitamin D that functions to regulate
many different cellular processes [5].