COL6A1 and ossification of the posterior longitudinal ligament of the spine: Other genes with increased expression of >50% included Sox9, COL11A2, COL6A1, and bone morphogenetic protein (BMP2) in OPLL sample case 1, PTHrP in OPLL sample case 2, and COL11A2, patched 1 (PTCH1), Gli3, and serine/threonine protein kinase 36 (STK36) in OPLL sample case 3.