Brugada syndrome is an inherited cardiac disorder caused by mutations in the cardiac sodium channel gene,SCN5A,that leads to ventricular fibrillation and sudden death. This study reports the changes in functional expression and cellular localization of anSCN5Adouble mutant (R1232W/T1620M) recently discovered in patients with Brugada syndrome. Mutant and wild-type (WT) human heart sodium channels (hNav1.5) were expressed in tsA201 cells in the presence of the &bgr;1-auxiliary subunit. Patch-clamp experiments in whole-cell configuration were conducted to assess functional expression. Immunohistochemistry and confocal microscopy were used to determine the spatial distribution of either WT or mutant cardiac sodium channels. The results show an abolition of functional sodium channel expression of the hNav1.5/R1232W/T1620M mutant in the tsA201 cells. A conservative positively charged mutant, hNav1.5/R1232K/T1620M, produced functional channels. Immunofluorescent staining showed that the FLAG-tagged hNav1.5/WT transfected into tsA201 cells was localized on the cell surface, whereas the FLAG-tagged hNav1.5/R1232W/T1620M mutant was colocalized with calnexin within the endoplasmic reticulum (ER). These results indicate that a positively charged arginine or lysine residue at position 1232 in the double mutant is required for the proper transport and functional expression of the hNav1.5 protein. These results support the concept that loss of function of the cardiac Na+channel is responsible for the Brugada syndrome. The full text of this article is available at http://www.circresaha.org.