Domain numbers under mechanical stress. (A) Schematic diagram of AFM-forced unfolding with a single repeat shown unfolded. Such unfolding yields a single peak (arrows) in the sawtooth-shaped force-extension plot; the first and last peaks (circles) correspond to desorption of the protein from either the surface or the AFM tip. (B) Frequency of occurrence for unfolding spectrin domains (N_pk-2). The mutant shows fewer high N_pk sawtooth patterns, indicating a relative loss of domain structure.