COL6A1 and ossification of the posterior longitudinal ligament of the spine: RT-qPCR analysis was performed using RNA extracted from peripheral blood cells (Fig. 2) and demonstrated that COL6A1 mRNA levels were ~ 6-fold higher in T-OPLL patients carrying the rs201153092A mutation than T-OPLL patients with the wild-type rs201153092G site (P < 0.01).