FSCB (fibrous sheath CABYR binding protein) is a testis- and sperm-specific protein primarily involved in late-stage fibrous sheath biogenesis and sperm function. FSCB is a post-meiotic protein expressed during spermatogenesis that localizes to the fibrous sheath cortex of the sperm flagellar principal piece 1. As a calcium-binding protein phosphorylated by protein kinase A, FSCB serves as a binding partner for CABYR, a calcium-binding protein phosphorylated during sperm capacitation 1. FSCB inhibits SUMOylation of ROPN1 and ROPN1L proteins, suggesting roles in post-translational protein modification. Beyond reproductive function, FSCB variants have been associated with ocular refraction and spherical equivalent, implicating this gene in refractive error development 2. Additionally, rare copy number variations involving FSCB have been identified in cerebral palsy cases, though the functional significance remains unclear 3. A somatic mutation in FSCB (S775L) was detected in osteosarcoma samples, though its pathogenic role requires further investigation 4. While FSCB's primary characterized function relates to spermatogenesis and sperm motility, emerging evidence suggests broader physiological roles warranting further study.