A 2.5 kb genomic fragment, encompassing a portion of exon 1 and all of exons 2 through 6, was replaced with a neo cassette. The deletion was designed to eliminate both the secreted and transmembrane porteins produced by the endogenous locus. Tnfsf13 protein was undetected by FACS analysis of homozygous mutant splenocytes. Real-time RT-PCR indicated that the levels of Tnfsf12 and Senp3 transcript, produced by loci in close proximity to Tnfsf13, were not affected by the targeting event.