The FSIP Family: Roles in Health and Cancer.

Fibrous sheath interacting proteins 1 and 2 (FSIP1 and FSIP2) are evolutionarily conserved testis-specific antigens, exclusively expressed in germ cells of adult human tissues, where they play essential roles in spermatogenesis and testicular development. Aberrant re-expression of FSIP1 and FSIP2, however, has been frequently reported in multiple malignancies, driving oncogenic processes including uncontrolled proliferation, invasion, migration, and metastasis, and correlating with unfavorable clinical outcomes. Their restricted expression in normal tissues, together with their consistent association with poor prognosis across cancer types, highlights their potential as diagnostic biomarkers, therapeutic targets, and prognostic indicators. This review summarizes the structural features and biological functions of the FSIP family, emphasizes recent advances in elucidating their regulatory roles in tumor-associated signaling pathways, and outlines the major challenges and future perspectives in this emerging field.