A puzzle in DNA biophysics.

In this issue, Lee et al. report the experimental temperature-dependence of the unzipping force for two natural DNA sequences. For both sequences, the curves show an anomaly at temperatures around 40 degrees C. In this brief contribution, we stress that the anomaly is not easily explained within the established theoretical models for the biophysics of DNA. As this puzzle questions our basic understanding of DNA, it must be resolved, most likely by a combination of additional experiments and new theoretical work.