The invention relates to compositions comprising double-stranded oligonucleotides of identical or different sequences and/or length, said oligonucleotides having sequences 3′N1N2 . . . Ni-1Ni . . . Nj5′ wherein—3′Ni . . . Nj5′ is half of a double-stranded 19-28 mer oligonucleotide of sequence complementary to a target nucleic acid sequence present in a living cell, and—3′N1 . . . Ni-15′ is a 3-50 mer overhang of sequence allowing oligomerization of said double-stranded oligonucleotide. Compositions of transfection comprising said oligonucleotide compositions and there used for therapeutical application.