A sequence within exon 23 beginning with nucleotide 51 was replaced with the corresponding human cDNA encoding the C terminus of the gene and a stop codon. A loxP flanked neo was inserted into intron 23 and subsequently deleted via transient cre expression, leaving a single loxP site. Immunoprecipitation and Western blotting confirmed successful targeting and the presence of a chimeric mouse-human protein in mutants.