Exon 1 was replaced by a modified version of exon 1 that encoded a "Flag-HA" epitope tag at the amino terminus of the gene product. A floxed neomycin cassette was also placed downstream of the exon. After targeting, the neo-cassette was removed by transient expression of cre recombinase. Protein expression was confirmed by immunoblot assays using anti-HA antibody. (J:141142)