CD38 and primary effusion lymphoma: PEL cells are derived from post-germinal center B cells and often express plasma-cell markers including CD38, CD138, CD319, and IRF4.9 Rituximab, an anti-CD20 monoclonal antibody, has been studied in upfront PEL treatment;8,9 however, because PEL generally does not express CD20, any contribution is likely to be indirect, perhaps from killing of other CD20-expressing KSHV-infected cells.