Spermatozoa are dependent on the proximal epididymis environment to complete their maturation. However, no single specific factor crucial for this process has been identified. Rnase9 and Rnase10 are expressed specifically in the murine proximal epididymis. Located on chromosome 14C1 only 28 kb apart, they do not show high homology with each other. An orthologue of both genes exist in the rat and human, and Train A (porcine orthologue) is the most abundant (8090%) secretory protein in the initial segment (IS). Testosterone replacement experiments revealed that Rnase9 expression is under direct/indirect regulation by androgens and Rnase10 expression is regulated by testicular factors other than androgen. We recently observed that spermatozoa from Rnase10 KO mice lacked association with glutinous extra-cellular material in the distal epididymis, were released as single vigorously motile cells into bicarbonate-free medium, displayed no tendency for head-to-head agglutination and lacked affinity to the oviductal epithelium. Failure to gain the site of fertilization was associated with a gradual loss of ADAM3 protein from sperm surface during epididymal transit. Spermatozoa from KO mice are also unable to establish tenacious associations with the zona pellucida, yet they were capable of fertilization in vitro. The murine epididymal secretome of Rnase10 was assessed by incubating isolated IS fragments, with [35S]-methionine. Accumulated methionine-labeled proteins were fractionated by 2D gel. In contrast to porcine Train A, murine Rnase10 secretion represents only 1.51.7% of the total IS secretion. Total RNA from IS of WT and Rnase10 KO mice was analysed using the Affymetrix GeneChip® Mouse Exon 1.0 ST Array. Rnase9 expression was increased 4.10 fold (P<0.05) as a result of the loss of Rnase10. Human Rnase9 is a sperm binding protein, whereas Train A does not interact directly with passing sperm. Therefore, Rnase9 may be an intermediary, involved in the stabilisation of ADAM3 to the sperm surface.