There was no significant difference in the relationship between Hb and FGF23 in BD Index and BD Sibling children (P = 0.01 and P = 0.03 respectively, test for interaction: P = 0.5); BD Index logeFGF23 = [18.65(SE 5.6)] − [5.82(SE 2.21)(logeHb)] − [0.04(SE 0.09)(age)] and BD Sibling logeFGF23 = [14.3(SE 3.82)] − [3.47(SE 1.54)(logeHb)] − [0.10(SE 0.03)(age)]. The gene discussed is GSTM1; the disease is Behcet disease.