The mRNA levels of Cyp27b1, which is responsible for the conversion of 25(OH)D into the active form 1,25(OH)2D in the kidney, were significantly higher in the vitamin D-deficiency − UV irradiation group than in the groups of vitamin D-repletion − UV irradiation or vitamin D-deficiency + UV irradiation (Fig. 4B). This evidence concerns the gene CYP27B1 and vitamin D deficiency.