His doctoral thesis research was performed in the laboratory of Prof. Michael A. Marletta, one of the key scientists responsible for the discovery of nitric oxide generation by mammalian cells. There, Dr. Stone purified and characterized the sensor for nitric oxide, the soluble form of guanylate cyclase. Using spectral and rapid-kinetic techniques, Dr. Stone discovered that this heme-based sensor has an unusual heme environment, previously unknown in mammalian physiology, which allows the protein to be responsive to physiologic levels of nitric oxide, while not interacting with molecular oxygen. He also directs the Cardiovascular Histology Core Facility at MGH, which serves as a central core laboratory for NIH-funded multicenter trials relating to cardiovascular diseases.