Focus issue: the evolution of complexity.

With genomic and proteomic data sets in hand, scientists are beginning to understand how variation in signaling proteins and signaling networks influences the function of signaling pathways to regulate cellular behavior. These analyses have provided insight into how proteins have evolved, how biological networks have evolved, and how organisms have evolved. Not only do these discoveries inform us about evolution of life at different levels, but they also have the potential to help us understand human disease and begin to engineer signaling networks to achieve specified tasks.