Response to comment on "Force-clamp spectroscopy monitors the folding trajectory of a single protein"

The mechanical stretching of ubiquitin chains creates a novel and well-defined starting point for observing folding trajectories. These initial conditions, which are never reached through chemical denaturation, add new physics to the folding problem. In contrast to chemical denaturation, mechanical unfolding and refolding of tandem modular proteins occur in vivo. It is therefore not likely that such events involve chain-tangling.