SPIRE1 is an actin nucleation factor that functions across multiple cellular compartments to regulate cytoskeletal dynamics and cellular processes. In the cytoplasm, SPIRE1 nucleates actin filaments and remains associated with the pointed end of growing filaments 1. It plays critical roles in intracellular vesicle transport, mediating endosome biogenesis through actin patch formation on early endosomes in conjunction with annexin A2 2. SPIRE1 is essential for female meiosis, required for asymmetric spindle positioning, cleavage furrow formation, and polar body extrusion 3. During exocytosis of Weibel-Palade bodies, SPIRE1 associates with myosin Vc to form actin rings that support von Willebrand factor release 4. In metastatic breast cancer, soft extracellular matrix promotes SPIRE1-dependent peri-mitochondrial F-actin formation, driving mitochondrial fission and metabolic reprogramming that confers chemotherapy resistance 5. Notably, SPIRE1 functions in the nucleus where DYRK1A-mediated phosphorylation at S482 regulates its activity in assembling F-actin on damaged chr18 to facilitate DNA double-strand break repair 6. Additionally, SPIRE1 participates in innate immune signaling downstream of dsRNA sensing [UniProt annotation]. Translation of SPIRE1 is regulated by the long non-coding RNA lnc-SMaRT through G-quadruplex-mediated mechanisms in muscle differentiation 7.