Variation on an Src-like theme.

The modularity of protein architecture and the diversity of protein domains hint at a vast combinatorial richness. But evolution appears to have been relatively conservative about selecting new combinations. When a particular grouping of domains within a polypeptide chain can perform a concerted function, that combination tends to reappear in multiple genomic contexts. In other words, once a molecular solution to a functional problem has emerged, it is reused rather than reinvented.