ENPP1 and ossification of the posterior longitudinal ligament of the spine: ENPP1 genetic testing should be considered in adults with any of the following: osteomalacia or early‐onset osteoporosis with low fasting serum phosphorus and low ratio of the tubular maximum reabsorption of phosphate to glomerular filtration rate, tumor‐induced osteomalacia without evidence of tumor, PHEX‐negative XLH, joint calcification, joint stiffness secondary to enthesopathy, OPLL, diffuse DISH, or fusion of cervical vertebrae.