RIGI and viral infectious disease: Although melanoma differentiation factor 5 (MDA5), which is a PRR and shares similar signaling features and structural homology as RIG-I, can functionally substitute for RIG-I in tree shrews to sense RNA viruses and induce type-1 interferon (IFN-1) response (37, 40), delayed IFN-1 expression renders tree shrews more susceptible to viral infection.