CYP27B1 and vitamin D deficiency: This concept is supported by the epidemiological studies showing Vitamin D deficiency is associated with higher incidence of cancers [20,21,22,23], for which the explanation was Vitamin D deficiency represented lower circulated 25(OH)D concentration, leading to the fewer converted 1α,25(OH)2D3 in the extra-renal tissues due to the fewer substrates for CYP27B1 to convert.