In children with chronic allograft nephropathy, FGF-23 levels increase with the degree of chronic allograft failure, preceding increase in PTH, suggesting that FGF-23 may be a more sensitive biomarker of post-transplant phosphorus dysregulation and continuation or reactivation of CKD–MBD (55). Here, FGF23 is linked to Crouzon syndrome-acanthosis nigricans syndrome.