Living cells display complex signal processing behaviors, many of which are mediated by
networks of proteins specialized for signal transduction. Here we focus on the question of
how the remarkably diverse array of eukaryotic signaling circuits may have evolved. Many of
the mechanisms that connect signaling proteins into networks are highly modular: The core
catalytic activity of a signaling protein is physically and functionally separable from
molecular domains or motifs that determine its linkage to both inputs and outputs. This high�…