CD30 ligand (CD30L) is a member of the TNF family which is present on a variety of cells, including normal resting В cells, granulocytes, eosinophils, a fraction of bone marrow precursors, subsets of activated macrophages, and T cells (both CD4+ and CD8+), and is highly expressed by thymic medullary epithelial cells. In vitro, the interaction of CD30L with its receptor (CD30) has been proved to induce either cell proliferation or cell death depending on the cell type receiving the signal and/or the microenviron-mental conditions in which the signal is provided. This suggests an important role for CD30L in the regulation of different immune functions, including thymic negative selection, and in the pathogenesis of some immune-mediated disorders.