Fibroblast growth factor-23 (FGF23), which is produced predominantly by osteoblasts and osteocytes, regulates renal tubular phosphate reabsorption, acts as a counter-regulatory hormone of active vitamin D, increases as a consequence of a decreased GFR, and thus plays a pivotal role in CKD-MBD (37, 38). The gene discussed is FGF23; the disease is Marchiafava-Bignami disease.