Molecules with high affinity and base-sequence specificity are required to control gene expression. In living cells this is usually achieved via specific binding of regulatory proteins to defined nucleic acid sequences (HClkne & Lancelot, 1982). It was recently shown that small RNAs could also participate in the regulation of protein synthesis at the level of translation (Coleman et al., 1984). We previously described a new family of molecules which could be used to achieve selective control of gene expression (Asseline et al., 1983, 1984~). These molecules involve an oligodeoxynucleotide covalently linked to an intercalating agent. The oligonucleotide should provide binding specificity by hybridizing to the complementary sequence of the nucleic acid target provided the latter is in a single-stranded configuration. The intercalcating agent might interact with this hybrid duplex structure provided the length of the linker is sufficient to allow for appropriate folding (see Fig. I).