Functional replacement of mouse Ctla4 with the human CTLA4 gene in vivo. (A) Normal appearance of secondary lymphoid organs in 1-year-old homozygous human CTLA4 knock-in mice. (B) Normal expression of activation markers among spleen CD4 and CD8 T cells. Data shown are dot plots of gated CD8 (top panels) and CD4 (bottom panels) T cells.