Oligonucleotide-directed triple helix formation represents a promising approach to block gene expression at the transcriptional level. We have previously shown [10] that a triple-helix-forming oligonucleotide was able to inhibit promoter function of reporter constructs in live cells, provided that the oligonucleotide was covalently linked to an intercalating agent which stabilizes triple-helical complexes. In order to demonstrate that this inhibitory effect was due to triple helix formation, we have mutated the oligonucleotide target site in the promoter of the interleukin-2 receptor alpha-chain gene. The mutated version of the promoter does not bind, and is not inhibited by the oligonucleotide, demonstrating that the observed inhibition of the wild-type promoter is indeed due to triple helix formation within cells.