Peter Agre, at Johns Hopkins University, discovers in 1992, while investigating a membrane protein with no apparent connection to water transport, that this protein — which he names aquaporin — forms specific channels through the cell membrane that allow the selective and extraordinarily rapid passage of water molecules, solving a long-standing physiological mystery: how cells manage to exchange water with their surroundings much faster than simple diffusion through the lipid membrane would allow. Roderick MacKinnon, at Rockefeller University, determines in 1998 the complete three-dimensional structure of a potassium channel — a membrane protein that allows the selective passage of potassium ions while blocking other similarly sized ions such as sodium — revealing with atomic precision the exact molecular mechanism by which the channel distinguishes between ions almost identical in size. Together, both discoveries establish the fundamental structural principles of selective transport across cell membranes — of water in Agre's case, of ions in MacKinnon's case — processes essential for kidney function, nerve impulse transmission, and virtually all cell physiology, and constitute the molecular basis on which numerous drugs targeting ion channels are designed, including treatments for cardiac arrhythmias and epilepsy.