A 0.35 kb region of exon 7 including the 5' splice acceptor site was replaced with a PGK-neo cassette via homologous recombination. Gene expression was absent in homozygous mutant animals as demonstrated by Northern blot analysis of spleen, bone marrow, and thymus samples using probes to exons 1-6 and exon 7. Immunohistochemical analysis of spleen sections from 4 week old homozygous mutant mice did not detect protein product. (J:50626)