In a series of pioneering experiments conducted here at the National Institutes of Health (NIH), Martin Rodbell and his colleagues discovered a mechanism that transformed our understanding of how cells respond to signals, by studying hormones—substances which have specific effects on cells' activity. This mechanism explains not only how light and odors are perceived, how signals travel between neurons in the brain, and how some diseases affect the body. When Martin Rodbell began to investigate the roots of hormone action, he could not have predicted the broad impact his findings would have.