RNA action inhibitor and use thereof

A method for suppressing the expression of a target gene, comprising:

• administering an RNA duplex to a subject or an expression system in vivo, in vitro or ex vivo, the RNA duplex comprising a passenger strand and a guide strand for a target RNA that is at least a portion of mRNA of the target gene,
• wherein both ends of the double-stranded RNA formed by pairing the passenger strand and the guide strand form blunt ends,
• wherein the passenger strand comprises two units represented by formula (2) at each of the 5′ and 3′ ends, and the guide strand comprises two units represented by formula (2) at the 3′ end,
• where X represents an oxygen atom or a sulfur atom.

A target RNA detection agent comprising an antisense strand for the target RNA, the antisense strand comprising:

• a hybridization region comprising units represented by formula (1) capable of specifically hybridizing to the target RNA;
• a stem region comprising units represented by formula (1), directly linked to 3′ end and 5′ end of the hybridization region, respectively, and hybridizing to form a loop around the hybridization region;
• a signal element indicating that the hybridization region has hybridized to the target RNA;
• and
• one or two diaminopurine-containing units having 2,6-diaminopurine as a base instead of one or two adenine-units having an adenine base in the hybridization region, the adenine-containing units being spaced one to three bases from the 3′ end and 5′ end of the hybridization region,
• where X represents an oxygen atom or a sulfur atom.