Infection of human dendritic cells (DCs) by negative-strand RNA viruses, such as Newcastle disease virus, leads to the induction of the IFNbeta gene, IFNB1, through the activation of the RNA helicase RIG-I, which is encoded by DDX58. Expression levels of IFNB1 and DDX58 in infected DCs showed positive correlations at the population and the single-cell levels. DDX58 has a common and potentially functional single nucleotide polymorphism, rs10813831 (A/G), encoding an Arg7Cys amino acid change in the RIG-I protein caspase recruitment domain (CARD). Quantitative RT-PCR analysis on Newcastle disease virus-infected primary DCs from 130 individuals revealed a significant association of the Arg7Cys single nucleotide polymorphism with increased IFNB1 and DDX58 transcription. Allelic imbalance analysis ruled out allele-specific DDX58 message levels and suggested that the observed association between Arg7Cys and IFNB1 and DDX58 transcription originated from a functional change in RIG-I due to the amino acid substitution in the CARD. DDX58 transfection experiments in 293T cells confirmed a biological functional difference between RIG-I 7Cys and the more common RIG-I 7Arg. Taken together, these data indicate that the innate immune response to viral infection of human cells is modified by a functional polymorphism in the RIG-I CARD.