There is now evidence for the existence of four-stranded human DNA, which has previously been theorized and synthesized. A group at the University of Cambridge in the United Kingdom have reportedly identified quadruple-helix human DNA. These “G-quadruplexes” occur in guanine-rich regions of the genome.  

The Cambridge group provides strong evidence that such four-helix DNA does exist in humans, and suggested that it may play a central role in human disease. Using an antibody that tightly binds to G-quadruplex structures, but not to traditional double-helix DNA it was found that it bound to many different sites on human chromosomes in cultured tumor cells.

Such structures should be easily detected using flow cytometry but has not been reported to date.

Nature News | Four-strand DNA structure found in cells