Porcupine plot of the principal mode of conformational variability calculated from an MD ensemble. (a) Although some aspects of the breathing motion of MDM2/apo indicated in Fig. 2(a) are recaptured from a 10 ns MD simulation, the porcupine plot here indicates that the motion may be more complex. (b) Orthogonal view of the principal mode of MDM2/p53. As in the CONCOORD analysis of MDM2/p53 in Fig. 2(b), this view is dominated by a pair of antiparallel vortices at each end of the molecule, indicative of a two-lobed flexing.