ENPP1 and ossification of the posterior longitudinal ligament of the spine: Although reports of OPLL affecting ARHR2 patients are scarce due to the rarity of ARHR2, tiptoe walking mouse (ttw), first recognized almost 40 years ago as a murine model of OPLL, was later found to possess biallelic ENPP1 loss of function mutations, genetically linking ENPP1 to OPLL [39].