A targeting vector composed of an amino acid substitution of an arginine to an alanine (R576A) in the 3'region of the VGF nerve growth factor inducible (Vgf) gene inserted a 5' flanking loxP site into the Vgf 5' UTR, and a 3' flanking loxP site and FRT-flanked neomycin selection cassette into the Vgf 3 UTR. (J:344295)