By homologous recombination in ES cells, endogenous DNA sequence for arginine 274 and serine 276 residues in exon 7 of Rela gene were replaced with codons for alanine at both locations followed by a neo cassette flanked by two loxP sites inserted into intron after Exon 7. A neo cassette was then deleted either in the recombined ES cells by transfection of cre-recombinase or in mice by crossing with splicer mice harboring a Cre transgene, leaving only single loxP sites. The presence of the mutation was confirmed by RT-PCR and DNA sequencing. Somewhat reduced amount mutant protein was translated.