PER–ARNT–SIM (PAS) domains are found in many bacterial proteins and were originally described as intracellular sensor modules. Subsequently, several structural studies reported PAS-like domains in extracellular regions of transmembrane receptors; however, sequence-derived PAS domain models did not confirm the structure-based classification. Other reports suggested that these extracellular modules are related to the Cache sensory domain superfamily. Indeed, Upadhyay et al. now show that so-called PAS-like domains belong to the Cache superfamily, which is homologous to, but distinct from, PAS domains. Using 3D structures of bacterial cell surface receptors, the authors constructed computational models to identify new proteins that contain Cache domains. Furthermore, their data suggest that this domain is the most common extracellular sensor module in many organisms, including important pathogens.