Testicular sperm are morphologically differentiated but are not progressively motile nor able to fertilize an egg. Post-testicular maturation requires exposure of spermatozoa to the microenvironment of the epididymal lumen. Spermatozoa undergo extensive changes in the epididymis, including enzymatic modifications, loss of pre-existing components and addition of new glycoproteins from epididymal secretions. These modifying proteins and enzymes are synthesized by epithelial cells lining the epididymal duct and secreted apically into the lumen, where they come into contact with, and may be absorbed onto, the sperm membranes. The proteins encoded by the genes in this cluster are synthesized and secreted by epididymal epithelial cells.