Electrical signaling in animal nerves and muscles is largely carried out by proteins in the superfamily of voltage-gated ion channels. These proteins are based on a single homologous domain, but different types exist as single-domain tetramers, two-domain dimers, or four-domain proteins that comprise the whole pore-forming structure. Four-domain channels are hypothesized to have evolved from a single-domain ancestor by two rounds of internal duplication. The role that a channel plays in a cell’s physiology is largely determined by its selectivity for specific ion species and by the stimulus that opens the channel — its method of ‘gating’. The voltage-gated sodium (Nav) and calcium channels (Cav), which drive the upstroke of action potentials and transduce electrical signals into cellular signals, respectively, both have the four-domain architecture, whereas voltage-gated potassium channels (Kv) have only one domain. Crystallographic studies have led to important discoveries about ion permeation and gating in the single domain Kv channels, but structural studies of the four-domain Nav and Cav channels have not achieved the same level of precision, leaving the atomic details of these important proteins in the dark. The recent discovery of and subsequent crystallographic work on a voltage-gated, sodium-selective, single-domain channel in bacteria (BacNav) was therefore greeted with excitement as a potential model of four-domain Nav channels.