SMCP is a 116 amino acid cytoplasmic protein that is found in the outer capsule that is associated with sperm mitochondria. Expressed specifically in spermatids of seminiferous tubules, SMCP is thought to be involved in the organization and stabilization of the helical sheath structure and may play a role in overall sperm motility. SMCP has a short N-terminal segment, a C-terminal lysine and several internal cysteines. Defects in the gene encoding SMCP may be a cause of male infertility due to both reduced sperm motility and an inability to pierce the zona pellucida of the female egg.