Objective: There is a TTX- and Cs+-resistant background Na+ leak conductance in many neurons, but the molecular identities of the conductance and its regulation have not been well understood. The purposes of this study were to identify the proteins forming this conductance and to understand the molecular mechanisms of its regulation. Methods: Using patch clamp recording, knockout mice and protein chemistry, we characterized the function of NALCN, a 24-transmembrane- spanning protein resembling NaV and CaV channels. Results: We found that NALCN forms a voltage-independent, non-selective cation channel permeable to Na+. Mice with targeted disruption of the Nalcn gene have severely disrupted respiratory rhythms and are neonatal lethal. Hippocampal neurons without NALCN lack the TTX-resistant Na+ leak current. This current is also activated by the neuropeptide substance P. The activation is through a G-protein-coupled receptor, but is independent of the activation of G-proteins and, instead, requires the Src family of tyrosine kinases. In the brain, NALCN is indirectly associated with UNC79 via the two proteins' direct interaction with UNC80. Conclusion: These studies suggest that the NALCN-UNC79- UNC80 complex forms a Na+-leak conductance. The activation of the channel by substance P represents a model for the G- protein-independent channel activation through G-protein- coupled receptors.