Waldenström Macroglobulinemia (WM) is a rare B cell malignancy defined by greater than 10% infiltration of lymphoplasmacytic cells in the bone marrow (BM) and a circulating monoclonal Immunoglobulin M (IgM), while its precursor state IgM monoclonal gammopathy of undetermined significance (MGUS) has <10% BM infiltration. The gene discussed is CD40LG; the disease is monoclonal gammopathy.