The SCN5A gene, encoding human cardiac-type voltage-dependent Na+ channels (Nav1.5), is essential in initiation of cardiac activity. Genetic defect of SCN5A channel has been associated with a variety of inherited cardiac arrhythmic disorders(Benson et al., 2003; Shimizu, 2005; Modell & Lehmann, 2006; Wolf & Berul, 2006). We characterized a novel SCN5A mutation, R878C, in the second pore segment of SCN5A identified in a three-generation Chinese family associated with variable clinical expression of sick sinus syndrome and asymptomatic Brugdada Syndrome. SCN5A mutations were generated using site-directed mutagenesis and heterologously expressed in HEK293 cells. Whole-cell voltage clamp was used to record current and subcellular localization was determined by confocal imaging. Heterologous expression the WT channel in HEK293 cells showed average peak current density 95±6.2 pA/pF (n=15), while the R878C cells (n=20) did not show any detectable current. There was also no detectable current of R878K (n=10) with lysine substituting arginine at amino acid 878, demonstrating that the positive change at 878 is not the only key defect for the mutant channel. Further immunocytochemistrical characterization revealed that mutant channel had normal trafficking to the plasma membrane, which suggested that the mutant channel produced a non-functional channel phenotype but had normal intracellular trafficking to the plasma membrane.