Cell-membrane-spanning potassium ion channel proteins regulate heartbeat and generate nerve impulses—they are essential to health. And for years, researchers have had a healthy debate about how ions move through these channels. X-ray crystallographic studies show a cylinder-shaped selectivity filter inside the channels, with four contiguous ion-binding sites. The X-ray picture and other studies suggest that potassium ions take turns with intervening water molecules on the narrow path through the channels. But a multi-institution group of scientists is challenging that view (Science 2014, DOI: 10.1126/science.1254840). Bert L. de Groot of Germany’s Max Planck Institute for Biophysical Chemistry, Ulrich Zachariae of Scotland’s University of Dundee, Oxford University’s Chen Song, and coworkers analyzed more than 1,300 potassium ion channel crossings using molecular dynamics simulations. They find that K+ ions move single file through the pore, with nary a water molecule. The team conducted its own analysis of X-ray data and concluded the data support direct ion-ion contact. In an accompanying commentary, Gerhard Hummer of the Max Planck Institute of Biophysics says that even though some inconsistencies with experimental data remain unresolved, researchers in doubt should not brush aside the new results.