H form or H-DNA
Although, the B form of DNA is the most stable form of DNA, duplex DNA can assume triplex form due to negatively superhelical stress. This form was first studied in a sequence from a spacer between the histone genes of sea urchin, and was described as H form, because it was stabilized by hydrogen ions (it was a protonated structure). This H form consisted of an intramolecular triple helix formed by the pyrimidine strand and half of purine strand, the other half of purine strand being single stranded.

Due to single stranded part of H form, it is hypersensitive to SI nuclease. Certain chemicals have also been shown to induce formation of H-form, in vitro. They can also penetrate living cells and facilitate formation of H form there. These chemicals include osmium tetroxide, chloroacetaldehyde and psoralen, which have occasionally been used for designing of TEOs for targeted mutagenesis.

It has also been shown in some cases that homopurine and homopyrimidine stretches are essential for promoter functioning. These sequences are targets for nuclear proteins, which are transcriptional activators. These features are suggestive of the formation of H-DNA in promoter regions, thus regulating transcription.