Similar to type I IFNs, type III IFNs, such as IFN-λ1 (IL-29), IFN-λ2 (IL-28A), IFN-λ3 (IL-28B), and IFN-λ4, are vital in the host’s defense against viral infections, with restricted expression on mucosal tissues (respiratory, gastrointestinal, and urogenital tracts). The gene discussed is IFNL1; the disease is viral infectious disease.