The invention provides a conjugate comprising (i) a nucleic acid which is complementary to a target nucleic acid sequence and which expression prevents or reduces expression of the target nucleic acid and (ii) a selectivity agent which is capable of binding with high affinity to a receptor which can be internalized by the cell in response to the binding of said selectivity agent. The conjugates of the present invention are useful for the delivery of the nucleic acid to cell of interests and thus, for the treatment of diseases which require a down-regulation of the protein encoded by the target nucleic acid as well as for the delivery of contrast agents to the cells for diagnostic purposes.