FCGR2B and systemic lupus erythematosus: We had previously found that in the FcγRIIB−/−Yaa and FcγRIIB−/− lupus models, the IRF5 heterozygotes were markedly protected from disease and exhibited a phenotype similar to the IRF5-deficient mice, even though IRF5 protein expression was still approximately 40% of the wildtype level [21].