Evolution reuses established modules. At the level of cell signaling, protein domains are used in many contexts to transfer different messages. A frequently occurring binding domain uses a structural scaffold to allow for sequence variation at critical sites without compromising structural stability. Even random mutations have a high chance of conferring a novel function, and only a small fraction of available sequence space is actually explored. Accordingly, current lab techniques allow us to infer evolutionary routes, exploring the possible and the attainable in terms of complex structure-function relationships.