A 2.5 kb genomic fragment containing exons 1b through 9 was replaced with a neomycin selection cassette inserted by homologous recombination. The deleted region encoded the initiator methionine and a portion of the Rel homology domain. Protein was undetected in homozygous mutant mice by Western blot analysis of spleen whole-cell protein extracts and a supershift analysis of spleen nuclear extracts. (J:77049)