SPEF1 (sperm flagellar and cilia associated 1) is a multifunctional microtubule-associated protein critical for ciliary structure and epithelial cell function. Structurally, SPEF1 contains an N-terminal calponin-homology domain for microtubule binding and a C-terminal coiled-coil region enabling homodimer formation 1. Its primary function involves stabilizing and bundling microtubules; specifically, SPEF1 binds to microtubule seams and crosslinks parallel microtubules to maintain structural integrity 2. In motile cilia, SPEF1 is essential for central pair apparatus formation, a '9+2' structure required for planar beating. SPEF1 depletion abolishes central pair microtubules and disrupts ciliary localization of proteins like HYDIN and SPAG6, resulting in rotational ciliary beats 1. WDR47 coordinates SPEF1 with JHY to stabilize central pair microtubules, with evolutionary evidence suggesting this mechanism emerged to enable fixed ciliary waveforms 3. Beyond cilia, SPEF1 localizes to microvillar structures in enterocytes where it limits apical actomyosin contractility, preserving intestinal barrier function by preventing hypercontractility-driven junction disruption 4. SPEF1 also binds actin and regulates cell migration through filopodia and lamellipodia formation 5, suggesting roles in planar cell polarity signaling. Clinical relevance includes potential association with malignant transformation in ciliated lesions 6.