Figure 1

Schematic showing how the history of a cluster can affect its construction. For a given cluster (1), two sequences (A) and (B) show significant identity. Depending upon which sequence is processed first, cluster 1A or cluster 1B can be constructed. Addition of further sequences showing identity to (A) or (B) then leads to the formation of different clusters (1A, 2A) or (1B, 2B) depending on whether cluster 1A or 1B was originally built.