Triplex-forming oligonucleotides (TFO) bind in the major groove of duplex DNA in a sequence-specific manner through the formation of non Watson-Crick (Hoogsteen) hydrogen bonds. The formation of a triplex along the major groove competes with the binding of transcription factors and other proteins that are necessary for transcription, thereby inhibiting the expression of particular genes. A variety of nucleoside analogues have been used in TFO - 8-amino-dG, 8-amino-dA, 6-thio-dG and deoxypseudouridine.