Voltage-gated Na⁺ channel (VGSC), predominantly neonatal Nav1.5 (nNav1.5), expression is upregulated in metastatic breast cancer (BCa) and VGSC activity potentiates a variety of metastatic cell behaviours (Fraser et al. 2005). VGSCs incorporate one or more β-subunit (VGSCβ) which can modulate channel gating and functional expression (Chen et al. 2002). Interestingly, VGSCβ’s can also serve as cell adhesion molecules by interacting with extracellular matrix proteins and cytoskeleton (Isom, 2002). The aims of the study were (1) to determine which VGSCβ’s were expressed in weakly and strongly metastatic human BCa (MCF-7 and MDA-MB-231, respectively) cell lines and (2) to test the possible role of VGSCβ expression in cellular adhesion and migration. MCF-7 and MDA-MB-231 cells were cultured (Fraser et al. 2005). Real-time PCR and Western blot (WB) with a polyclonal antibody specific for β1 (Malhotra et al. 2000) were used to study and quantify gene and protein expression. Transwell migration and adhesion were measured as before (Fraser et al. 2005; Aydar et al. 2006). A small interfering RNA (siRNA) technique was used to silence specifically the β1 gene in MCF-7 cells; the control transfection involved a scrambled siRNA (siControl). Data (mean ± SEM) were analysed by paired t tests, unless otherwise stated. Real-time PCR revealed that in both MCF-7 and MDA-MB-231 cell lines, VGSCβ expression profile was as follows: β1>> β4> β2 (β3 was absent). MCF-7 cells had a much higher overall level of VGSCβ mRNA expression, compared to MDA-MB-231. WBs verified that β1 protein was strongly expressed in MCF-7 and barely detectable in MDA-MB-231 cells. In siRNA-transfected MCF-7 cells, β1 mRNA (normalised to cytochrome b5 reductase) was reduced by 76 ± 4% after 4 days, relative to siControl (P<0.01; n=3). Protein-level reduction (normalised to actin) was less pronounced, only 39 ± 7% after 8 days (P<0.01; n=11). Under control conditions, MCF-7 cells were much more adhesive than MDA-MB-231 cells, in line with their weak metastatic potential. However, 8 days after β1 siRNA transfection, the adhesion of MCF-7 cells was reduced by 34 ± 2% (P<0.001; n=6). Interestingly, concurrently, the cells’ migratory activity increased by 121 ± 14% compared to siControls (P < 0.05; n = 8). This increase was largely dependent upon VGSC activity since application of TTX (10 microM) to siRNA-treated cells reduced migration to the control level (P= 0.4 for siRNA vs siControl, both treated with TTX; n=8). Indeed, in siRNA-treated MCF-7 cells, nNav1.5 mRNA and protein levels were significantly higher. We conclude (1) that VGSCβ1 subunit expression makes a significant contribution to the strong adhesiveness of the weakly metastatic MCF-7 cells and (2) that VGSCβ1 downregulation would increase migration via VGSC expression/activity.