The amino acid sequence of AK155 is homologous to that of IL-10 and AK155 belongs to a growing family of viral and cellular IL-10 homologs. Together with the genes for IFN and IL-22/IL-TIF, ak155 maps to the human chromosomal region 12q15. The ak155 gene is one of the very few differentially expressed genes by human T cells after growth transformation with herpesvirus saimiri (saimiriine herpesvirus type 2), a tumor virus of neotropical squirrel monkeys (Saimiri sciureus). Only herpesvirus saimiritransformed T cells have been found to overexpress ak155 and to release the protein into the tissueculture supernatant. ak155 is transcribed at low levels in a series of other T cell lines and in primary cells isolated from peripheral blood cells, but it is not detectable in B cells. Similarly to IL-10, AK155 forms homodimers. AK155 is proposed to contribute tothe transformed phenotype of human T cells after infection by herpesvirus saimiri. Moreover, AK155 is highly likely to play a pathophysiological role in hematology or immunology.