GC and rheumatoid arthritis: Further suggestions of a link between vitamin D and RA come from studies reporting associations with SNPs from the vitamin D pathway genes VDR (144) (encoding vitamin D receptor), GC (145) (encoding vitamin D-binding protein) and CYP27B1 (146) (encoding 25-hydroxy-vitamin D-activating enzyme 1α-hydroxylase) (137, 139).