While CXCL9 has been established as an important biomarker for IFN-γ activity in Familial HLH (FHL) and secondary HLH, very little is understood about the location, kinetics, and quality of the tissue responses to IFN-γ and the functional role of CXCL9 in pathogenesis in HLH20,28. The gene discussed is IFNG; the disease is hemophagocytic syndrome.