Human fHLH has been modeled in PRF1 deficient mice infected with LCMV, and both CD8 T cells and interferon-gamma (IFNγ), a cytokine known to be the main driver of anemia in models of fHLH and fulminant MAS (32, 33), were found to be critically important mediators of mouse mortality (34). The gene discussed is IFNG; the disease is macrophage activation syndrome.