BMP4 and ossification of the posterior longitudinal ligament of the spine: Previous studies have shown that single nucleotide polymorphisms (SNPs) in the transforming growth factor-β, nucleotide pyrophosphatase (NPPS), leptin receptor, collagen 6A1(COL6A1), bone morphogenetic protein 2 (BMP2) and bone morphogenetic protein 4 (BMP4) as well as estrogen receptor (ER) and interleukin-1 (IL-1) genes are associated with the development of OPLL [17], [18], [19], [20], [21].