Proteins located in the sperm fibrous sheath may be useful target to develop effective immunotherapeutic strategies. (A) Schematic diagram showing the structure of the human spermatozoon. (B) Electron micrograph showing a cross-section of the sperm fibrous sheath. The FS is a unique cytoskeletal structure of the spermatozoon. The cancer-testis antigen Sp17 belongs to the fibrous sheath proteins family and has been found in several tumors of unrelated histological origin, including benign pituitary adenomas (C) and primary ovarian carcinomas (D).