Homologous recombination inserted a floxed neomycin-thymidine kinase expression cassette downstream of the exon that encodes amino acids 236-258, which is part of the lumenal "stem" domain, and introduced an additional loxP site upstream of the exon. Cre-mediated recombination in ES cells excised the neo-tk sequences, leaving loxP sites flanking the exon. (J:41621)