Quadruple helices that intertwine four, rather than two, DNA strands had been made in the laboratory, but were regarded as curiosities as there was no evidence that they existed in nature. Now, they have been identified in a range of human cancer cells.

The four-stranded packages of DNA, dubbed G-quadruplexes, are formed by the interaction of four guanine bases that together form a square. They appear to be transitory structures, and were most abundant when cells were poised to divide. They appeared in the core of chromosomes and also in telomeres, the caps on the tips of chromosomes that protect them from damage.

Because cancer cells divide so rapidly, and often have defects in their telomeres, the quadruple helix might be a feature unique to cancer cells. If so, any treatments that target them will not harm healthy cells.

"I hope our discovery challenges the dogma that we really understand DNA structure because Watson and Crick solved it in 1953," says Shankar Balasubramanian of the University of Cambridge, UK.