Exon 6 encoding the high-affinity DNA-binding domain was flanked with loxP sites by homologous recombination of a construct that also contained a PGK-neo and thymidine kinase selection markers. The PGK-neo-thymidine kinase marker was excised by transient transfection of properly targeted ES cells with a cre-expressing plasmid and left exon 6 flanked by loxP sites. (J:93481)