The presence of enzymes CYP2R1 and CYP27B1 and the detection of vitamin D receptor expression in testicular tissue as well as evidence for clinically relevant vitamin D deficiency in hypogonadal men [17, 23, 27–29] suggested a prominent role of the testis in the metabolism of this vitamin [15, 16, 19, 24]. Here, CYP27B1 is linked to vitamin D deficiency.