The invention relates to a synthetic double-stranded oligonucleotide capable of modifying the molecular phenotype of osteoclasts and increasing the expression of the oestrogen alpha receptor gene. Pharmaceutical compositions comprising the oligonucleotide according to the invention are also described, as well as therapeutic applications of that oligonucleotide, in particular for the treatment of osteopenic diseases such as for example osteoporosis. The oligonucleotide according to the invention is characterised in that it comprises the sequence 5'-ATTTATTTTCAATACTGACT-3' or a fragment or a mutant thereof.