ENPP1 and ossification of the posterior longitudinal ligament of the spine: Previous clinical sequencing results have shown that the expression of the ENPP1 gene in the peripheral blood of OPLL patients was significantly lower than that of normal individuals (Saito et al., 2011; Mehta et al., 2012; Ferreira et al., 2021; Kato et al., 2022), and naturally mutated tiptoe walking (ttw) mice have been shown to have mutations in the gene site of Enpp1 (Mackenzie et al., 2012).