VDR and vitamin D deficiency: These insights clarify why vitamin D deficiency is possibly linked to RA and is associated with higher disease activity and a worse quality of life [8,9].Vitamin D mediates its function by binding to the vitamin D receptor (VDR), which is encoded by the VDR gene—a member of nuclear hormone receptors superfamily detected in various immune cell types that is also present on chondrocytes and synovial cells in RA affected joints [5,10].