Recent work has indicated that RNA interference (RNAi) is a natural, conserved mechanism of antiviral immunity in plants, vertebrates, and insects. RNAi is an RNA-dependent gene silencing process triggered by a long double-stranded RNA (dsRNA). When dsRNA is introduced into a cell, a specific RNaseIII endonuclease, Dicer, binds and cleaves dsRNA to produce double-stranded fragments of 20-25 base pairs with 2-nt 3? overhangs, called small interfering RNAs (siRNAs). The siRNAs are integrated into the RNA-induced silencing complex (RISC) to activate the RISC. Activated RISC bind to homologous mRNA and cause sequence-specific degradation of the target mRNA. Positive-stranded RNA viruses are vulnerable to RNAi because the viruses replicate their genomes through complementary strands resulting in dsRNA replication intermediates. Since the genomes of most honey bee viruses are positive-stranded RNA molecules, RNAi is an important defense mechanism against viruses in honey bees.