IL-17 is a cytokine secreted in large amounts exclusively by T cells upon activation, which acts directly on stromal cells and induces their production of several proinflammatory and hematopoietic bioac-tive molecules. A functional homolog of IL-17 is present in the genome of the T lymphotropic herpesvirus saimiri (HVS). A receptor of low affinity for IL-17 has been cloned and is ubiquitously expressed.