The one-step “dimerization” (with loss of ethylene) of theimmunosuppressant FK506 by olefin metathesishas been used to create new chemical inducers of dimerization (CIDs).These small-molecule protein dimerizers areshown to activate signal transduction pathways and gene transcriptionat low concentrations in mammalian cells.Their ease of synthesis provides ready access to reagents ofwidespread utility in studies of the cellular functions ofproteins.