= Solution
Reusable <protein domains> make evolution combinatorial: a new domain combination can join an existing catalytic activity to a binding, localization or regulatory module without inventing every component anew. <Gene duplication> allows one copy to retain an established role while another diverges; <domain shuffling> and fusion can create new combinations. Connecting domains can also increase effective local <concentration> of interacting sites or enable <allostery>.
These are potential advantages, not guarantees that every fusion is beneficial: domains must still fold and work together. The domain-level analysis behind this example found extensive reuse and new combinations of recognized structural units.<Exploring structural diversity across the protein universe with The Encyclopedia of Domains by Andy M. Lau and others>[Domain classification and inter-domain packing relationships]{ref}
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