Generation of IL-21R knock-out mice. (A) The genomic structure of the IL-21 receptor gene, and the targeting vector used for homologous recombination, resulting in the disruption of exons 2 to 5 of the wild-type gene. (B) Mice were genotyped by PCR with the wild-type gene yielding a 503-bp product and the knock-out gene yielding a 752-bp product. (C) IL-21R mRNA from splenocytes of wild-type and IL-21R^−/− mice was assessed by RT-PCR.