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. A sequence cassette encoding 1. an attP site, 2. a loxP cassette containing the wGMR.PH marker has been inserted in place of the deleted sequence.