The Y-linked autoimmune accelerating (Yaa) locus drives the transition to fatal lupus nephritis when combined with B6. Sle1 in our C57BL/6J (B6)-congenic model of systemic autoimmunity. We and others recently demonstrated that the translocation of a cluster of X-linked genes onto the Y chromosome is the genetic lesion underlying Yaa (Subramanian, S. et al., Proc. Natl. Acad. Sci. USA 2006. 103: 9970–9975; Pisitkun, P. et al., Science 2006. 312: 1669–1672). In male mice carrying Yaa, the transcription of several genes within the translocated segment is increased roughly twofold. Although the translocated X chromosome segment in Yaa may contain as many as 16 genes, the major candidate gene for causation of the Yaa-associated autoimmune phenotypes has been TLR7. To confirm the role of TLR7 in Yaa-mediated autoimmune phenotypes, we introgressed a targeted disruption of TLR7 (TLR7–) onto B6. Sle1Yaa to produce B6. Sle1YaaTLR7–and examined evidence of disease at 6 and 9 months of age. Our results demonstrate that the up-regulation of TLR7 in the B6. Sle1Yaa strain is responsible for splenomegaly, glomerular nephritis and the majority of the cellular abnormalities of B, T and myeloid cells. The upregulation of TLR7 was also responsible for driving the infiltration and activation of leukocytes in the kidney, in which activated T cells were a primary component. However, the resolution of TLR7 up-regulation did not eliminate the enhanced humoral autoimmunity observed in B6. SleYaa, suggesting that additional elements in the translocation may contribute to the disease phenotype.