Genome-wide studies have identified associations between polymorphisms in the IFN regulatory factor-5 (Irf5) gene and a variety of human autoimmune diseases. Its functional role in disease pathogenesis, however, remains unclear, as studies in Irf5^−/− mice have reached disparate conclusions regarding the importance of this transcription factor in type I IFN production and antibody responses. We identified a spontaneous genomic duplication and frameshift mutation in the guanine exchange factor dedicator of cytokinesis 2 (Dock2) that has arisen in at least a subset of circulating Irf5^−/− mice and inadvertently been bred to homozygosity. Retroviral expression of DOCK2, but not IRF-5, rescued defects in plasmacytoid dendritic cell and B-cell development, and Irf5^−/− mice lacking the mutation in Dock2 exhibited normal plasmacytoid dendritic cell and B-cell development, largely intact type I IFN responses, and relatively normal antibody responses to viral infection. Thus, confirmation of the normal Dock2 genotype in circulating Irf5^−/− mice is warranted, and our data may partly explain conflicting results in this field.