Interleukin-26 (IL-26), initially termed AK155, is a cellular sequence homolog to IL-10 belonging to the IL-10 cytokine family. Together with the related genes for interferon-gamma and IL-22/IL-TIF, il-26 maps to the human chromosomal region 12q15. The il-26 gene is one of the few differentially expressed genes specifying human T cells after growth-transformation with herpesvirus saimiri, a tumor virus of neo-tropical squirrel monkeys. Only herpesvirus saimiri-transformed T cells have been found to strongly over-express il-26 and to release the protein into the tissue culture supernatant. In a series of other T-cell lines and in native peripheral blood cells, il-26 is transcribed at low levels, but it is not detectable in B cells. Similarly to IL-10, the IL-26 protein forms homo-dimers. IL-26 is a candidate to contribute to the transformed phenotype of human T cells after infection by herpesvirus saimiri. Moreover, the T-lymphokine IL-26 is highly likely to play a role in normal and pathological hematology or immunology.