FNBP4 (formin binding protein 4) is a eukaryotic nuclear protein that functions as a regulator of actin cytoskeleton dynamics through interaction with formin proteins 1. The protein contains two WW domains that mediate protein-protein interactions 2, and interacts specifically with the FH1 domain and interdomain connector region of FMN1 (formin 1) 1. FNBP4 acts as a stationary inhibitor of FMN1-mediated actin assembly, preventing FMN1 from displacing capping proteins at actin filament barbed ends and inhibiting bundling activity in a concentration-dependent manner 1. The protein contains a monopartite nuclear localization signal, indicating its exclusive nuclear localization and suggesting a potential role in regulating nuclear actin dynamics 1. FNBP4 has emerging clinical relevance as an autoantigen. Anti-FNBP4 autoantibodies are elevated in relapsing polychondritis (RP) patients and decrease from acute attack to remission, suggesting involvement in disease pathogenesis 3. Additionally, anti-FNBP4 is part of a diagnostic panel for Ro/SSA-negative Sjögren's disease patients, contributing to disease detection and differentiation from healthy controls 4. FNBP4 has also been identified in gene fusion events associated with myofibroma (MTCH2-FNBP4 fusion) 5, indicating broader pathological significance.