The mouse cyritestin gene is a member of the ADAM (a disintegrin and metalloprotease) gene family and codes for a membrane-anchored sperm protein. Recently, it was shown that cyritestin is critical for male fertility in the mouse. Spermatozoa of cyritestin-deficient mice are not able to bind to the zona pellucida of the oocyte and therefore unable to fertilize the egg. However, zona-free oocytes can be fertilized and the resulting embryos show normal development. In contrast to the mouse, where only one gene for cyritestin (Cyrn) is reported, two cyritestin genes (CYRN1 and CYRN2) are known in humans. The human CYRN1 and CYRN2 genes are located on chromosomes 8 and 16, respectively. We report that 27% of fertile men are deficient for the CYRN1 gene but that all have a CYRN2 gene, suggesting that the CYRN2 gene is the orthologous mouse cyritestin gene in humans and might be involved in sperm-egg interactions. However, the characterization of CYRN2 transcripts from testicular RNA of CYRN1-deficient men demonstrated many termination codons in the synthesized cyritestin cDNA. Furthermore, Western-blot analysis with human testicular protein extracts using an anti-cyritestin antibody failed to detect any cyritestin protein. These results demonstrate clearly that both cyritestin genes are non-functional in humans.