FGF21 and Menkes disease: When considering the MD cohort alone, we neither found any statistical correlation between pGSN and the previously reported biomarkers (pGSN and FGF-21: rs = −0.161, p = 0.24; pGSN and GDF-15: rs = −0.001, p = 0.99), nor between FGF-21 and GDF-15 (rs = 0.105, p = 0.44).