Using a combination of database screening and polymerase chain reaction analyses we have determined the sequence and structure of the human sperm tail associated gene SPAG4. SPAG4 is a 12-exon gene spanning approximately 5.2 kb on chromosome 20q11.23. It is expressed in a limited number of normal tissues, notably the pancreas and testis, but is switched on and greatly upregulated in a wide range of neoplastic tissues. The data therefore strongly suggests that SPAG4 expression is a potential clinically relevant cancer marker.