Sodium plays a key role in determining the basal excitability of the nervous systems through the resting "leak" Na(+) permeabilities, but the molecular identities of the TTX- and Cs(+)-resistant Na(+) leak conductance are totally unknown. Here we show that this conductance is formed by the protein NALCN, a substantially uncharacterized member of the sodium/calcium channel family. Unlike any of the other 20 family members, NALCN forms a voltage-independent, nonselective cation channel. NALCN mutant mice have a severely disrupted respiratory rhythm and die within 24 hours of birth. Brain stem-spinal cord recordings reveal reduced neuronal firing. The TTX- and Cs(+)-resistant background Na(+) leak current is absent in the mutant hippocampal neurons. The resting membrane potentials of the mutant neurons are relatively insensitive to changes in extracellular Na(+) concentration. Thus, NALCN, a nonselective cation channel, forms the background Na(+) leak conductance and controls neuronal excitability.