A floxed neo cassette was inserted into exon 9 via homologous recombination resulting in a premature stop codon between the DNA-binding domain and the CTA domain. The neo cassette was removed by germ line, cre-mediated recombination. Removal of the C-terminal domain results in a 40% to 50% reduction in transcriptional activation activity and the production of a truncated protein. (J:137876)