IFNL4 (interferon lambda 4) is a type III interferon that activates the JAK-STAT signaling pathway to upregulate antiviral genes 1. However, IFNL4 exhibits atypical immunological behavior compared to canonical interferons. While the protein itself activates antiviral responses indistinguishable from other interferon lambda family members in vitro 2, the IFNL4 gene is poorly induced by viral infection due to its noncanonical promoter structure lacking effective interferon regulatory factor (IRF) binding sites 2. IFNL4 expression is controlled by transcription factors Sp1, NF-κB, IRF3, and IRF7 3. Genetically, IFNL4 expression depends on the rs368234815 frameshift deletion polymorphism; the ancestral ΔG allele produces functional IFNL4 protein, while the TT variant causes premature termination 1. Paradoxically, individuals carrying the functional IFNL4-ΔG allele show impaired hepatitis C virus clearance and increased liver inflammation and fibrosis 4, suggesting IFNL4 acts in a proviral and anti-inflammatory manner 2. The null IFNL4 genotype confers protection against HIV-1 infection 5 and is associated with severe COVID-19 outcomes 6. Strong evolutionary selection favors the null allele in European and Asian populations, while the ancestral ΔG allele predominates in African populations 1.