Surface roughness is defined by a Smoothed Atomic Fractal Dimension (SAFD) method described previously (Pettit, F.K. & Bowie, J.U. (1999), J. Mol. Biol. 285, 1377-1382). Briefly, we place a sphere of constant radius Rf = 8Å around each exposed atom and sum accessible surface areas of neighboring atoms within the sphere. This sum of accessible areas depends on the solvent probe radius RW. Next, for each atom, we estimate the derivative of its area-sum-within-sphere as RW varies. The derivative is estimated from the difference between area sums at slightly different values of RW. Here, RW varies from 2.1 Å to 2.2 Å.