Sperm mitochondria differ in morphology and subcellular localization from those of somatic cells. They are elongated, flattened, and arranged circumferentially to form a helical coiled sheath in the midpiece of the sperm flagellum. Sperm mitochondrial-associated cysteine-rich protein is a protein localizes to the capsule associated with the mitochondrial outer membranes and is thought to function in the organization and stabilization of the helical structure of the sperm's mitochondrial sheath.The striking homology lies in the dicysteine motifs. Northern and Southern zooblot analyses revealed that the MCSP gene in human, baboon, and bovine is more conserved than its counterparts in mouse and rat. Northern blot and in situ hybridization experiments demonstrated that expression of the human MCSP gene is restricted to haploid spermatids.