The sequence within the first exon that encodes the first 200 amino acids of the protein has been replaced with a green fluorescent protein (GFP) cDNA that includes an ATG translation termination codon and a polyadenylation signal. GFP is therefore expressed under control of the endogenous promoter, and the endogenous protein is not expressed. A tk-neo cassette that was flanked by loxP sites in the original construct was deleted in vivo.