In individuals under 50 yrs (Table 3), pGSN concentrations under 252 μg/mL best differentiated MD patients vs. healthy controls with 73.53% sensitivity and 100% specificity; for FGF-21, a threshold value of 225 pg/mL diagnosed MDs with 65.71% sensitivity and 90.91% specificity, and for GDF-15, a threshold value of 634 pg/mL diagnosed MDs with 85.29% sensitivity and 90.91% specificity. The gene discussed is FGF21; the disease is Menkes disease.