Mixed cryoglobulinemia vasculitis (MCV) is a lymphoproliferative disorder of marginal zone B-cells, producing polyreactive IgM endowed with rheumatoid factor activity, which are nonmalignant but are prone to neoplastic transformation; the pathogenic IgM forms cryoprecipitable immune complexes with endogenous IgG that in vivo cause a small vessel vasculitis (1). The gene discussed is CD40LG; the disease is lymphoproliferative syndrome.