Sperm Equatorial Segment Protein 1 (SPESP1) is a member of the SPESP1 family. SPESP1 is highly expressed in the testis, where it is localized to the acrosome of postmeiotic stages of spermiogenesis; it is expressed at lower levels in the placenta and fetal lung. SPESP1 is involved in the multicellular organisimal development. Disruption of SPESP1 leads to abnormal distribution of sperm proteins resulting in a detached membrane from the equatorial segment and less fertile sperm. SPESP1 may interact with IZUMO1 and MN9 antigen and it contains an N-glycosylation site as well as several cAMP-dependent kinase, protein kinase C, and casein kinase II consensus phosphorylation sites.