Acrosomal protein SP-1, also known as Acrosomal vesicle protein 1 and ACRV1, is a testis-specific, differentiation antigen, that arises within the acrosomal vesicle during spermatogenesis, and is associated with the acrosomal membranes and matrix of mature sperm. Regulation of cell type-specific gene transcription is central to cellular differentiation and development. During spermatogenesis, a number of testis-specific genes are expressed in a precise spatiotemporal order. The longest transcript of ACRV1 / SP-1 is the most abundant, comprising 53-72% of the total acrosomal vesicle protein 1 messages; the second largest transcript comprises 15-32%; the third and the fourth largest transcripts account for 3.4-8.3% and 8.7-12.5%, respectively; and the remaining transcripts combined account for < 1% of the total acrosomal vesicle protein 1 message. ACRV1 / SP-1 is a testis-specific acrosomal protein that has been detected in several species including humans. It may be involved in sperm-zona binding or penetration, and it is a potential contraceptive vaccine immunogen for humans. ACRV1 / SP-1 may be involved in sperm-zona binding or penetration. It is also a intra-acrosomal protein that is considered to be a vaccine candidate for immunocontraception.