Entropy profiles. Each structural class corresponds to a particular position in the hexapeptide motif found in the LβH proteins. Structural classes i and i+4 are the most conserved while the other classes present a more variable composition. SCPE profiles correspond to equilibrium amino acid distributions (see Methods). The SCPE parameters were fit to the minimum of the entropy error (see Figure 5). The profile obtained from JTT simulations of 0.28 substitutions per site is shown for comparison.