IFNλ4 expression is suppressed during viral infections. IFNλ4p179 has similar antiviral properties as IFNλ3, signaling exclusively through the extracellular IFNλR1 and IL-10R2 heterodimer to activate STAT1 and induce ISGs. During viral infections, several mechanisms are in place that suppress the expression of the functional IFNL4 isoform compared with robust IFNL3 expression.