A floxed neo cassette and loxP site were introduced flanking exon 2 via homologous recombination. Transient expression of cre-recombinase in correctly targeted ES cells resulted in deletion of exon 2 and the neo cassette leaving behind a single loxP site. Absence of protein product in homozygous mutant animals was confirmed by Western blot anlysis of heart, spleen, and lung extracts. (J:82490)