COL6A1 and ossification of the posterior longitudinal ligament of the spine: To date, 16 genes/loci that are linked to OPLL susceptibility have been reported [5], including toll-like receptor 5 (TLR5) [6], collagen type XII α2 chain (COL11A2) [7], COL6A1 [8–10], runt-related transcription factor 2 (RUNX2) [11], bone morphogenetic protein 2 (BMP2) [12], bone morphogenetic protein 4 (BMP4) [13], transforming growth factor β1 (TGFB1) [14], interleukin 1β (IL1B) [15], and interleukin 15 receptor subunit α (IL15RA) [16].