“…20 Various strategies have been proposed to model backbone flexibility in design, such as small random perturbations in torsional or Cartesian space, 7,21-23 normal mode analysis, 24 or "backrub" motions shown to underlie commonly observed protein structural heterogeneity in high resolution crystal structures 6 that have proven useful to model structural changes in response to mutations. 8,25,26 Several strategies have used protein backbone ensembles for design, which are typically generated computationally employing various sampling methods 21,22,27 including backrub motions [28][29][30][31] but can also be taken from crystal structures. 23,28,31 Within the structure modeling and design program Rosetta, 32 backbone flexibility has been treated in a number of ways.…”