The loxP cassette containing the wGMR.PH marker has been excised, resulting in a chromosome in which the N 5' region, including the promoter region, all known transcription initiation sites, and the first exon including the 5'UTR, the signal peptide sequence, and the subsequent nine amino acid residues, as well as about 1 kb of the first intron have been deleted, and a single attP site and a single loxP site are present in place of the deleted sequence.