The percentage of BD children with an elevated C-FGF23 concentration (>125 RU/ml) decreased from 100 to 25% (BD geometric mean (−1s.d., +1s.d.) C-FGF23: from 536 (167, 1715) to 85 (53, 140) RU/ml; conditional regression P=0.01) (Fig. 1A). Here, FGF23 is linked to Behcet disease.