Roderick MacKinnon (1956- present) was awarded half the 2003 Nobel Prize in Chemistry for his structural and mechanistic studies of ion channels. Before MacKinnon's discoveries, the detailed molecular architecture of ion channels and how they convey ions in and out of cells were unclear. MacKinnon and colleagues studied "filters" in channels that passed one type of ion while blocking others. By using X-ray crystallography, he determined the three-dimensional molecular structure of an ion channel. The channel, MacKinnon discovered, easily strips potassium ions—but not sodium ions—of their associated water molecules and allows them to slip through to the inside of the cell. He also found a molecular "sensor" in the end of the channel nearest the cell's interior that reacts to conditions around the cell, sending signals that open and close the channel at the appropriate times.