To evaluate the in vivo activation of the IFNγ pathway in patients with sHLH/MAS and to investigate whether levels of activated STAT1 represent a biomarker for the identification of patients at early stages of the disease, we measured basal activated Tyrosine (701) phosphorylated STAT1 (pSTAT1) levels in monocytes of patients with sHLH/MAS at time of blood sampling. Here, STAT1 is linked to macrophage activation syndrome.