Waldenström’s macroglobulinemia (WM) is an incurable low-grade lymphoplasmacytic lymphoma, characterized by bone marrow (BM) infiltration of small, IgM-positive lymphocytes with varying degrees of plasmacytoid or plasma cell differentiation and the presence of monoclonal immunoglobulin M (IgM) paraproteins (M-spikes) in the serum (1–5). The gene discussed is CD40LG; the disease is lymphoplasmacytic lymphoma.