VDR and osteoporosis: Several candidate genes and their SNPs are strongly associated with osteoporosis pathogenesis, including transforming growth factor-β1 (TGF-β1), interleukin-10 (IL-10), vitamin D receptor (VDR), osteoprotegerin (OPG), collagen type I α 1 (COL1A1) and estrogen receptor (ER) genes, and may act as potential clinical markers for osteoporosis [7,8,9].