Based on our previous data obtained in a mouse model of MAS, showing that the up-regulation in the mRNA expression levels of IFNG and IFNγ-inducible genes in liver and spleen was associated with an increase in phosphorylated STAT1 protein levels [8], we evaluated Tyr(701)-phosphorylated STAT1 levels in liver biopsies. The gene discussed is STAT1; the disease is macrophage activation syndrome.