Sp17 was originally proposed to be a sperm-specific protein and thought to play a role in sperm-egg interactions by binding to the zona pellucida via two conserved heparin-binding motifs. However, more recent data indicate that it may be expressed more widely, both in tumours and in normal somatic tissues. The complete nucleotide and deduced amino acid sequences of human Sp17 transcripts are reported and it is shown that they are expressed in a wide range of tissues, albeit at a much lower expression than in the testis. On the basis of the extremely high sequence conservation throughout the N-terminal half of Sp17, and the presence within this region of an A-kinase anchoring protein (AKAP)-binding motif, it is postulated that the proposed role of Sp17 in zona pellucida binding is unlikely to be its principal function.