FGF21 and Menkes disease: When comparing young MD vs. young non-MD patients (Figure 3E and Table 2), the diagnostic value for pGSN (AUC = 0.67, IC95% (0.51; 0.80)) and GDF-15 (AUC = 0.85, IC95% (0.72; 0.98)) markedly improved relative to the global cohorts (Figure 2E); however, the diagnostic value of FGF-21 remained stable (AUC = 0.78, IC95% (0.63; 0.92)) regardless of the age range.