We have previously isolated the testatin gene using a modified mRNA differential display method on RNA from developing male and female mouse gonads. This gene is specifically expressed during early testis development, immediately after the onset of the testis-determining gene SRY: The protein encoded by testatin has features that are characteristic for type 2 cystatins, a family of small inhibitors of cystein proteases such as the cathepsins. We have now isolated the human orthologue of this gene. We describe here the sequence, genomic structure, chromosomal location, and expression pattern of the human testatin gene. Like mouse testatin, human testatin is specifically expressed in the testis, suggesting that it has a function in reproduction. We have therefore also investigated whether the human testatin gene plays a role in disorders of gonadal development, by sequencing the gene in patients with gonadal dysgenesis, with true hermaphroditism, and in children with less well-defined intersex conditions. We found no sequence aberrations in these patients apart from an H109P polymorphism which was also found in fertile controls. This is the first genetic analysis of testatin in humans.