STAT1 and macrophage activation syndrome: Based on several data demonstrating that, in vivo and ex vivo, glucocorticoids inhibits IFNγ-mediated phosphorylation of STAT1 (9, 22), we divided sHLH/MAS patients into two groups: patients without any treatment at time of blood sampling (untreated patients, n=10) and patients who were already receiving glucocorticoids (treated patients, n=14).