First, positive correlations of HGS parameters with hemoglobin levels observed in both PCS cohorts, with ACE2 in the PCS/non-ME/CFS cohort, and with bilirubin and ferritin in the PCS/ME/CFS cohort as well as negative correlations with NT-proBNP levels in the PCS/ME/CFS cohort point to endothelial dysfunction and hypoperfusion as cause of muscle fatigue. The gene discussed is NPPB; the disease is endothelial dysfunction.