A team of researchers at Cambridge has discovered the presence of G-quadruplex DNA in human cells using antibody-specific binding. The G- quadruplex forms when DNA is rich in guanine and is highly thermodynamically stable, which explains how it can occur in vivo. Quadruplex DNA is observed most often during S phase after DNA has replicated. This structure has been previously observed in ciliates, but never before in mammals.
Source: Biffi G, Tannahill D, McCafferty J, Balasubramanian S 2013. Quantitative visualization of DNA G-quadruplex structures in human cells. Nat Chem advance online publication. doi: http://www.nature.com/nchem/journal/vaop/ncurrent/abs/nchem.1548.html#supplementary-information