Two loxP sites were introduced flanking exons 2 to 8 and another set of exons 2-8 containing a T to A change resulting in a cysteine to serine substitution at amino acid 247 (C247S) in exon 5 was incorporated after the second loxP site. In addition, a neomycin selection cassette flanked by self-deletion anchor sites was inserted upstream of the second loxP site and was removed in germ cells. (J:322003)