VDR and intervertebral disk degenerative disorder: Association studies of genes encoding for structural and functional components of intervertebral disc have highlighted the participation of polymorphisms in IDD [18, 20–22], including collagens I [23], IX [24], and XI [25], aggrecan [26], cartilage intermediate layer protein (CILP) [27], ECM-degrading enzymes such as MMP-3 [28] and MMP9, thrombospondin-2 (THBS2) [29], inflammatory cytokines interleukin-1 alpha (IL-1α) [30], IL-18 [21], IL-6, and tumor necrosis factor alpha (TNF-α) [31], and vitamin D receptor (VDR) [32].