Vitamin D aids bone health not only by helping to prevent secondary hyperparathyroidism, but also because calcitriol, produced from circulating 25-hydroxyvitamin D by 25-hydroxyvitamin D 1-α-hydroxylase (CYP27B1) in osteoblasts, complements the transcriptional activity of RUNX2 when bound to the vitamin D receptor; most notably, it collaborates with RUNX2 in promoting the expression of osteocalcin (OC), a crucial mediator of hydroxyapatite deposition [243,244,245,246,247]. Here, RUNX2 is linked to secondary hyperparathyroidism.