The part of exon 3 was replaced with a mouse Oxtr cDNA tagged with the hemagglutinin epitope at the 3' end (Oxtr CDNA), internal ribosomal entery site (IRES) and cre, and a FRT flanked pgk-neo cassette was inserted in intron 3 via homologous recombination. The neo cassette was then removed by flippase mediated recombination. (J:287190)