Similar to the MYD88L265P mutation, which alone is not an absolute diagnostic marker for Waldenstrom’s macroglobulinemia and is found in 50–80% of patients with IgM monoclonal gammopathy of uncertain significance (58), STAT3 mutations alone are unlikely to allow a reliable diagnosis of T-LGL leukemia when clonal T-LGL proliferation is found. The gene discussed is CD40LG; the disease is Waldenstrom macroglobulinemia.