Regulatory subunit of multiple voltage-gated sodium channel complexes that play important roles in excitable membranes in brain, heart and skeletal muscle (PubMed:1375395, PubMed:8282123, PubMed:8021275, PubMed:15178439, PubMed:28012039). Enhances the presence of the pore-forming alpha subunit at the cell surface and modulates channel gating characteristics and the rate of channel inactivation (PubMed:1375395, PubMed:8282123, PubMed:8021275, PubMed:15178439, PubMed:28012039). Modulates the activity of a variety of pore-forming alpha subunits, such as SCN1A, SCN2A, SCN3A, SCN4A, SCN5A and SCN10A (PubMed:1375395, PubMed:8282123, PubMed:8021275, PubMed:15178439, PubMed:28012039). Component of a voltage-sensitive sodium channel complex that consists of a pore-forming alpha subunit and one or more regulatory beta subunits (PubMed:1375395, PubMed:28012039). Interacts with SCN4A (PubMed:28012039). Interacts with NFASC (PubMed:11470829). Interacts with SCN10A (PubMed:15178439). Interacts with SCN1A. Interacts with SCN3A. Interacts with SCN5A. Interacts with SCN8A (By similarity). Detected at nodes of Ranvier on the sciatic nerve. Detected in brain (at protein level) (PubMed:11470829). Expressed in brain, heart, skeletal muscle and spinal cord (PubMed:1375395, PubMed:8282123). In developing nodes of Ranvier, it is localized in the sciatic nerve at postnatal days 3 and 10, during the process of myelination and maturation of the nodes. Belongs to the sodium channel auxiliary subunit SCN1B (TC 8.A.17) family.