Abstract Body: Voltage-gated sodium channels, crucial for the action potential upstroke in cardiomyocytes, consist primarily of a single pore-forming α-subunit and 1 or 2 accessory β-subunits. SCN1B encodes two variants: β1, a transmembrane cell adhesion molecule, and β1B, typically described as a soluble secreted protein. While β1 and β1B share identical sequences in the amino-terminal (NT) extracellular Ig domain, they differ in their carboxy-terminal (CT) sequence. The Ig domain of β1/β1B has been deemed critical in maintaining normal cardiac conduction by various groups. Traditionally, effects on cardiac conduction have been attributed to β1, despite difficulties in detecting it at the protein level in the heart. We hypothesized that β1B, rather than β1, plays a significant role in electrical activity propagation in the heart. Utilizing an NT antibody (detects both variants), a β1-specific CT antibody, and a custom β1B-specific CT antibody, we examined SCN1B gene product profiles in various cardiac cell types, including neonatal rat ventricular myocytes and fibroblasts (NRVMs and NFs), isolated adult mouse cardiomyocytes (MCMs), and tissue slices from isolated adult rat hearts. The β1-specific CT antibody failed to label any bands in Western Blots (WB) of these cells and tissues, whereas the NT antibody detected a 37 kD band in all cell types and showed high levels of localization at cell-cell contacts using immunofluorescence (IF) with NRVMs. Conversely, the CT antibody showed minimal immunolabeling evidence. Moreover, an inhibitor of β1 Regulated Intramembrane Proteolysis failed to generate the expected 19 kD fragment if β1 were present in NRVMs. In adult rat hearts, the NT antibody labeled intercalated discs, while the CT showed minimal signal. Conversely, in WBs of NRVMs using our custom β1B antibody, we observed a 37 kD band consistent with the presence of β1B, and in heart sections, we found labeling of intercalated discs. These findings suggest that the major β subunit in working ventricular myocytes may be β1B, not β1. Further investigation is warranted, as these results could significantly impact our understanding of the roles and functional effects of β1 and β1B on cardiac conduction.