The lin-12/Notch proteins appear to act as transmembrane receptors for intercellular signals that specify cell fates during animal development. Recent structure/function studies have shown that the lin-12/Notch intracellular domain alone has the intrinsic signal-transducing activity of the intact protein, and that the role of the extracellular domain is to regulate this intrinsic activity. These studies have also suggested that the different lin-12/Notch proteins in a given organism are interchangeable biochemically and have addressed the role of lin-12/Notch genes in development.