The one-step "dimerization" (with loss of ethylene) of the immunosuppressant FK506 by olefin metathesis has been used to create new chemical inducers of dimerization (CIDs). These small-molecule protein dimerizers are shown to activate signal transduction pathways and gene transcription at low concentrations in mammalian cells. Their ease of synthesis provides ready access to reagents of widespread utility in studies of the cellular functions of proteins.