Analysis of the complete BMP9 gene on single markers and haplotypes in 450 patients with OPLL and in 550 matched controls and subsequent linkage disequilibrium (LD) analysis identified one 3-kb block of intense LD in BMP9 and one specific haplotype CTCA, which should contain the OPLL-associated risk alleles and be considered as a risk factor for OPLL [60]. Here, GDF2 is linked to ossification of the posterior longitudinal ligament of the spine.