CABS1 (calcium binding protein, spermatid associated 1) is a multifunctional calcium-binding protein with primary roles in male reproductive function and broader immunoregulatory activities. In spermatogenesis, CABS1 localizes to the sperm flagella, particularly the principal piece and fibrous sheath, where it is essential for maintaining structural integrity and progressive motility 1. CABS1 deficiency causes defective fibrous sheath architecture, abnormal microtubule configuration, and impaired calcium regulation, leading to subfertility and potentially asthenozoospermia in humans 1. Additionally, CABS1 is crucial for acrosome formation and proper localization of the oocyte activation factor PLCζ; mutations in CABS1 result in total fertilization failure and teratozoospermia 2. Beyond reproduction, CABS1 exhibits tissue-wide distribution in salivary glands, immune system, and other organs 3, and contains a primate-specific anti-inflammatory heptapeptide domain near its C-terminus 4. In human saliva, CABS1 levels correlate with psychological stress and inflammatory markers, with lower molecular weight forms associated with stress resilience 5. Structurally, CABS1 is a highly disordered protein with predicted ligand-binding capacity for Ca²⁺, Mg²⁺, and Zn²⁺, suggesting roles in energy metabolism and mitochondrial function 4. Dysregulation of CABS1 expression contributes to metastatic prostate cancer progression 6.