Covalent attachment of intercalating agents to triplex-forming oligonucleotides (TFOs) is a promising strategy to enhance triplex stability and biological activity. We have explored the possibility to use the anticancer drug daunomycin as triplex stabilizing agent. Daunomycin-conjugated TFOs (dauno-TFOs) bind with high affinity and maintain the sequence-specificity required for targeting individual genes in the human genome. Here, we examined the effects of two dauno-TFOs targeting the c-myc gene on gene expression, cell proliferation and survival. The dauno-TFOs were directed to sequences immediately upstream (dauno-GT11A) and downstream (dauno-GT11B) the major transcriptional start site in the c-myc gene. Both dauno-TFOs were able to down-regulate promoter activity and transcription of the endogenous gene. Myc-targeted dauno-TFOs inhibited growth and induced apoptosis of prostate cancer cells constitutively expressing the gene. Daunomycin-conjugated control oligonucleotides with similar sequences had only minimal effects, confirming that the activity of dauno-TFOs was sequence-specific and triplex-mediated. To test the selectivity of dauno-TFOs, we examined their effects on growth of normal human fibroblasts, which express low levels of c-myc. Despite their ability to inhibit c-myc transcription, both dauno-TFOs failed to inhibit growth of normal fibroblasts at concentrations that inhibited growth of prostate cancer cells. In contrast, daunomycin inhibited equally fibroblasts and prostate cancer cells. Thus, daunomycin per se did not contribute to the antiproliferative activity of dauno-TFOs, although it greatly enhanced their ability to form stable triplexes at the target sites and down-regulate c-myc. Our data indicate that dauno-TFOs are attractive gene-targeting agents for development of new cancer therapeutics.