In healthy subjects, serum CXCL7 levels were > 11 ng/mL; however, patients with the circumscribed and segmental types of OPLL showed levels of approximately 4 ng/mL; patients with the mixed type of OPLL showed levels of approximately 3.3 ng/mL, and patients with the severe continuous type showed levels of < 2 ng/mL. Here, PPBP is linked to ossification of the posterior longitudinal ligament of the spine.