Structures of the 6 sample domains. The ribbon rendering of the structure of the proteins of Fig. 1: (a) d1s99a_, a 2-fold symmetric ferredoxin-like fold, (b) d1wd3a2, a 3-fold symmetric beta-trefoil, (c) d1jofc_, a 7-bladed beta-propeller, (d) d1vzwa1, a 2-fold symmetric TIM barrel, (e) d1bk5a_, an alpha/alpha superhelix, and (f) d2j8ka1, a right-handed beta-helix with square cross-section. The ribbons are colored in rainbow colors, starting from blue for the N-terminus and ending with red at the C-terminus. The calculated symmetry axis is shown as a black rod with a ball at the center in this and in all other figures.