TYK2 and viral infectious disease: As the first line of defense against viral infection, type I interferon response is initiated after IFNs bind to heterodimeric receptors, consisting of interferon-α receptors 1 (IFNAR1) and 2 (IFNAR2) on the cell surface, leading to the activation of the tyrosine kinase 2 (Tyk2) and Janus kinase 1 (Jak1).