As far as we know, studies that carefully addressed expression profiles of vitamin D metabolic genes and key proteins like the vitamin D receptor (VDR), CYP27B1 (1α-hydroxylase, vitamin D activating enzyme) and CYP24A1 (24-hydroxylase, vitamin D inactivation) in COPD (and controls) are lacking. Here, CYP27B1 is linked to chronic obstructive pulmonary disease.