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Summary: Antiviral immunity is triggered by immunorecognition of viral nucleic acids. The cytosolic helicase RIG-I is a key sensor of viral infections and is activated by RNA containing a triphosphate at the 5′ end. The exact structure of RNA activating RIG-I remains controversial. Here, we established a chemical approach for 5′ triphosphate oligoribonucleotide synthesis and found that synthetic single-stranded 5′ triphosphate oligoribonucleotides were unable to bind and activate RIG-I. Conversely, the addition of the synthetic complementary strand resulted in optimal binding and activation of RIG-I. Short double-strand conformation with base pairing of the nucleoside carrying the 5′ triphosphate was required. RIG-I activation was impaired by a 3′ overhang at the 5′ triphosphate end. These results define the structure of RNA for full RIG-I activation and explain how RIG-I detects negative-strand RNA viruses that lack long double-stranded RNA but do contain blunt short double-stranded 5′ triphosphate RNA in the panhandle region of their single-stranded genome. [Copyright &y& Elsevier]