Anti-ESP antibodies enable researchers to detect and measure the ESP antigen in biological samples. This target is a reported synonym of the SPESP1 gene, which encodes sperm equatorial segment protein 1. This protein is known to be involved in fertilization ability of sperm. The human version of ESP has a canonical amino acid length of 350 residues and a protein mass of 38.9 kilodaltons. It is reported to be localized in the cytoplasmic vesicles of cells and notably expressed in the testis. ESP is a member of the SPESP1 protein family.