Exon 3 was flanked with a neo cassette and loxP sites via homologous recombination. Transient expression of cre recombinase in correctly targeted ES cells facilitated removal of the neo cassette and exon 3 leaving behind a single loxP site in its place. Exon 3 encodes the C-terminal portion of the homeodomain and the remainder of the protein. (J:57314)