Figure 1 Alignment of representative PCI domains. Only the better-conserved C-terminal half of the complete PCI domain is shown. Only sequences matching established profiles with an error probability of P 0.01 were accepted during iterative profile refinement and are shown here. Error probabilities were estimated as described previously12. Identical amino acid residues are shown against a black background; residues that are similar in at least 50% of the sequences are shown against a grey background. Numbers on the left indicate the position of each domain within the protein sequence. Alternative names for these proteins can be found in Table I. The sequences are grouped by biological function, which is indicated by the colour bars on the left: magenta, proteasome; green, COP9; cyan, eIF3. The red letters at the top of the alignment indicate the positions of the predicted-helices8. HS, human; MM, mouse; SC, Saccharomyces cerevisiae; AT, Arabidopsis thaliana.