G-protein–coupled receptors (GPCRs) are a large and diverse family of seven-transmembrane–domain proteins expressed on the surface of human cells. These molecules respond to external stimuli by initiating signal-transduction pathways that affect the expression of a large family of genes — which, in turn, regulate a range of vital physiological processes and functions. Figure 1 illustrates the general pathways of GPCRs. Without these proteins, humans simply could not survive: Without β-adrenergic receptors, we could not regulate our blood sugar, for example;…