A 10.9 kb region containing exons 1 through 5 was replaced by a neomycin selection cassette inserted at the endogenous locus via homologous recombination. The deleted exons encoded 213 amino acids, including the intitiator methionine. RT-PCR and Western blot analyses of thymus extracts showed an absence of endogenous transcript and protein in homozygous mutant mice. (J:61085)