The invention relates to compositions comprising double-stranded oligonucleotides of identical or different sequences and/or length, said oligonucleotides having sequences .sup.3'N.sub.1N.sub.2 . . . N.sub.i-1N.sub.i . . . N.sub.j.sup.5' wherein--.sup.3N.sub.i . . . N.sub.j.sup.5' 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--.sup.3'N.sub.1 . . . N.sub.i-1.sup.5' is a 3-50 mer overhang of sequence allowing oligomerisation of said double-stranded oligonucleotide. Compositions of transfection comprising said oligonucleotide compositions and there used for therapeutical application.