FMN1 (formin 1) is a non-diaphanous formin family protein that regulates actin cytoskeleton dynamics and plays essential roles in development and cellular organization 1. The protein consists of conserved FMH1, FMH2, FH1, and FH2 domains, with the FMH2 domain functioning as an alpha-catenin binding region 1. FMN1 promotes actin filament assembly and bundling, and unlike diaphanous-related formins, it is not subject to canonical autoinhibition through DID/DAD domains or Rho GTPase activation 2. The protein's activity is negatively regulated by FNBP4, which acts as a stationary inhibitor by binding to the FH1 domain and interdomain connector, preventing FMN1 from displacing capping proteins at actin filament barbed ends 2. Clinically, mutations in the FMN1/GREM1 locus are associated with Cenani-Lenz syndrome, characterized by oligosyndactyly and metacarpal abnormalities, through effects on the BMP signaling pathway 3. Additionally, genetic variants in the FMN1/GREM1 region interact with body mass index in colorectal cancer risk, suggesting a role in cancer susceptibility 4. FMN1 serves as a scaffold protein interacting with multiple cellular components including profilin, SRC, and various FNBP proteins 1.