Keeping DNA Flexible The elastic behavior of DNA is important to biological processes that involve DNA bending and looping. However, there has been considerable debate over the flexibility of DNA at lengths below the persistence length (around 150 base pairs). A widely used approximation, the wormlike chain model, predicts stiff short DNA. Vafabakhsh and Ha (p. 1097 ; see the Perspective by Nelson ) directly monitored cyclization of single molecules of DNA, by using a fluorescence assay, and found significant looping, with the looping rate having only weak length dependence between 67 and 105 bp, which is inconsistent with the wormlike chain model. Instead, DNA binding proteins may stabilize transiently bent or looped DNA conformations.