c-Jun is an oncoprotein that comprises a portion of the AP-1 transcription factor and belongs to the basic-leucine zipper (bZIP) DNA binding protein family. Using peptides derived from the leucine zipper region of Fos, we have developed agents that inhibit Jun's DNA binding in the low micromolar range. Fos peptides that were effective inhibitors in the DNA binding assay were also found to inhibit cellular Jun binding to an AP-1 site in a luciferase reporter plasmid in MCF-7 cells. Size exclusion studies confirmed that peptides that inhibit the DNA binding of Jun also inhibit its dimerization. These peptides were found to have a cytotoxic effect on the MCF-7 cell line when delivered with the transfecting agent Tfx-50, possibly due to their role as transcription factor regulators.