Knowledge of spatial patterning of GTPases is critical to understanding protein function since subcellular localization is essential for normal protein function. In this paper, we discuss how Hahn's group has created a new type of genetically encoded, light-activated enzyme that allows precise temporal and spatial control of the small GTPase, Rac1. This reagent enables this group to analyze the roles of effector proteins in Rac1 function and may be applicable to other signaling proteins.