Potassium channels demonstrate a seemingly strange activity: they permit the passage of potassium ions, whereas they do not allow the passage of the much smaller sodium ions. Before MacKinnon's work, the detailed molecular architecture of potassium channels and the exact means by which they conduct ions remained speculative. In 1998, despite barriers to the structural study of integral membrane proteins that had stopped most attempts for decades, MacKinnon and colleagues determined the three-dimensional molecular structure of a potassium channel from bacteria utilizing X-ray crystallography. With this structure and other biochemical experiments, MacKinnon and colleagues were able to explain the exact mechanism by which potassium channel selectivity occurs.