GDF2 and ossification of the posterior longitudinal ligament of the spine: Extensive linkage and association studies have identified multiple genes/loci that link to OPLL susceptibility, including ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1)5, collagen 11A2 (COL11A2)6, collagen 6A1 (COL6A1)7, bone morphogenetic protein 2 (BMP2)8, bone morphogenetic protein 4 (BMP4)9, bone morphogenetic protein 9 (BMP9)10, transforming growth factor-β1 (TGFB1)11, transforming growth factor-β3 (TGFB3)12, transforming growth factor β receptor type 2 (TGFBR2)13 and fibroblast growth factor receptor 1 (FGFR1)14 and estrogen receptor 1 (ESR1)15.