Human autoimmune diseases thought to arise from the combined effects of multiple susceptibility genes include systemic lupus erythematosus (SLE) and autoimmune diabetes. Well-characterised polygenic mouse models closely resembling each of these diseases exist, and genetic evidence links receptors for the Fc portion of immunoglobulin G (FcR) with their pathogenesis in mice and humans [1-3]. FcRs may be activatory or inhibitory and regulate a variety of Immune and inflammatory processes [4,5]. FcγRII (CD32) negatively regulates activation of cells including B cells and macrophages . FcγRII-deficient mice are prone to immune mediated disease [7-9]. The gene encoding Fc gamma RII, Fcgr2, is contained in genetic susceptibility Intervals in mouse models of SLE such as the New Zealand Black (NZB) contribution to the (NZB x New Zealand White (NZW)) F1 strain [1,10,11] and the BXSB strain , and in human SLE [1-3]. We therefore sequenced Fcgr2 and identified a haplotype defined by deletions in the Fcgr2 promoter region that is present in major SLE-prone mouse strains (NZB, BXSB, SB/Le, MRL, 129 ) and non-obese diabetic (NOD) mice but absent in control strains (BALB/c, C57BL/6, DBA/2, C57BL/10) and NZW mice. The autoimmune haplotype was associated with reduced cell-surface expression of FcγRII on macrophages and activated B cells and with hyperactive macrophages resembling those of Fc gamma RII-deficient mice, and is therefore likely to play an important role in the pathogenesis of SLE and possibly diabetes.