CD30 was originally described as a marker of Hodgkin's and Reed–Sternberg cells in Hodgkin's lymphoma. Cloning and characterization of cDNAs encoding CD30 and its ligand (CD30L) established these proteins as members of the tumor necrosis factor receptor (TNFR) and tumor necrosis factor (TNF) superfamilies, respectively. Expression of CD30 is mostly restricted to virus–infected lymphocytes, neoplasms of lymphoid origin and a subset of activated T cells which produce Th2–type cytokines. The ligand is present on activated T cells, resting B cells, granulocytes, and the medulla of the thymus (epithelial cells and Hassal's corpuscles) as well as in various leukemia cells. The biological function of CD30 is pleiotropic but has come to be understood in the context of co–stimulatory signals. Signal transduction of CD30 utilizes signal transducers, TNFR–associated factors (TRAF1, 2, 3 and 5), which are shared by other TNFR family members. Mechanisms of signal transduction leading to diverse biological functions remain to be elucidated.