Voltage-sensing domains (VSDs) of voltage-gated ion channels sense changes in the membrane potential and as a result alter the conduction state of the channel. The human voltage-gated sodium channel NaV1.5 is primarily expressed in cardiac muscle and is responsible for the rising phase of the cardiac action potential. Mutations within NaV1.5 can lead to fatal cardiac arrhythmias. Such mutations have been found throughout the gene, including missense mutations within the VSD of repeat IV that have been shown to lead to Brugada Syndrome and LQT3. The VSDs also play an important role as binding sites for gating modifier peptide toxins from tarantula spider venoms. Such toxins could serve as good lead compounds for drug development due to their high specificity and more subtle mode of action compared to pore blockers. Therefore, it would be important to know the structures of the human sodium channels VSDs. While no structures of whole eukaryotic sodium channel proteins exist, isolated VSDs of other ion channels have been shown to fold into their native conformation in the absence of the pore forming domain. Therefore, we are pursuing the expression and purification of the isolated VSDs of human sodium channels in order to investigate the structural changes within the VSD caused by pathogenic mutations and by the binding of gating-modifier toxins. Here, we present the expression and purification of the human NaV1.5 VSD of repeat IV in a bacterial expression system in isotopically labeled form and preliminary characterization of the truncated protein. NaV1.5 VSD IV is expressed in Escherichia coli in minimal media, and extracted from membranes by solubilization into n-dodecylphosphocholine micelles. Purified NaV1.5 VSD IV was characterized by mass spectrometry and gel filtration chromatography and used for preliminary NMR structural studies.