Abstract
At first glance, signaling molecules that interact with cell-surface receptors would seem to have only two choices--to bind to the receptor to turn it on or dissociate to turn it off. But then how can one explain the diverse responses elicited by molecules that interact with the same receptor? In his Perspective, Malissen discusses new findings, including two reports in this week's issue by [Torigoe][1] and [Neumeister Kersch][2], that suggest a molecular solution to this mystery. [][3] [1]: http://www.sciencemag.org/cgi/content/short/281/5376/568 [2]: http://www.sciencemag.org/cgi/content/short/281/5376/572 [3]: http://