Pumping Out Sodium Mammalian cells contain relatively high concentrations of potassium but low concentrations of sodium. This balance is maintained by an ion pump, the Na + , K + –adenosine triphosphatase, in an adenosine triphosphate–driven transport cycle that results in the export of three sodium ions and the import of two potassium ions. Structures of potassium-bound conformations of the pump have been determined. Now, Nyblom et al. (p. 123 , published online 19 September) report on the high-resolution crystal structure of a Na + -bound conformation, which reveals conformational changes associated with Na + binding.