Figure 1.

Histogram of all amino-acid sequence identities. The histogram N_a(p) (panel A) and the normalized histogram n_a(p) = N_a(p)/[N_genes(N_genes - 1)/2] (panel B) of amino acid sequence identities p for all pairs of paralogous proteins in complete genomes of H. pylori (blue stars), E. coli (green open squares), S. cerevisiae (red crosses), C. elegans (cyan open triangles), D. melanogaster (magenta filled circles) and H. sapiens (brown filled triangles). The dashed line is a power-law p^-4. Note the logarithmic scale of both axes. Vertical lines separate regions I, II and III described in the text.