A composition comprising a first and a second nucleic acid complexes each comprising a first, a second, a third, and a fourth nucleic acid strands, each of the strands comprising, sequentially, a first, a second and a third fragments, wherein the nucleic strands are defined as: B-X-D, C - X - A, D - Y - E and F-Y-C for the first, second, third and fourth strands of the first complex, respectively, and C - Y - F, E-Y-D, F-Z-G, H - Z - E for the first, second, third and fourth strands of the second complex, respectively, wherein each letter denotes a fragment and each string of letters connected by "-" denotes a strand, and wherein: each of the first and the second complexes comprises a first duplex region formed between the second fragments of the first and second strands (X:: X and Y::Y, in the first and second complexes, respectively), a second duplex region formed between the second fragments of the third and fourth strands (Y:: Y and Z:: Z), a third duplex region formed between the third fragment of the first strand and the first fragment of the third strand (D:: D and F::F), and a fourth duplex region formed between the first fragment of the second strand and the third fragment of the fourth strand ( C::C and E:: E); in each of the first and second complexes, the first fragment (B and C, in the first and second complexes, respectively) of the first strand, the third fragment ( A and D) of the second strand, the third fragment ( E and G) of the third strand and the first fragment (F and H) of the fourth strand are single-stranded; the third strand of the first complex ( D - Y - E) has sequence complementarity to the second strand of the second complex (E-Y-D) to allow binding therebetween; and the fourth strand of the first complex (F-Y-C) has sequence complementarity to the first strand of the second complex ( C - Y - F) to allow binding therebetween. B-X-D, C - X - A, D - Y - E and F-Y-C for the first, second, third and fourth strands of the first complex, respectively, and C - Y - F, E-Y-D, F-Z-G, H - Z - E for the first, second, third and fourth strands of the second complex, respectively, wherein each letter denotes a fragment and each string of letters connected by "-" denotes a strand, each of the first and the second complexes comprises a first duplex region formed between the second fragments of the first and second strands (X:: X and Y::Y, in the first and second complexes, respectively), a second duplex region formed between the second fragments of the third and fourth strands (Y:: Y and Z:: Z), a third duplex region formed between the third fragment of the first strand and the first fragment of the third strand (D:: D and F::F), and a fourth duplex region formed between the first fragment of the second strand and the third fragment of the fourth strand ( C::C and E:: E); in each of the first and second complexes, the first fragment (B and C, in the first and second complexes, respectively) of the first strand, the third fragment ( A and D) of the second strand, the third fragment ( E and G) of the third strand and the first fragment (F and H) of the fourth strand are single-stranded; the third strand of the first complex ( D - Y - E) has sequence complementarity to the second strand of the second complex (E-Y-D) to allow binding therebetween; and the fourth strand of the first complex (F-Y-C) has sequence complementarity to the first strand of the second complex ( C - Y - F) to allow binding therebetween.