A loxP site was inserted upstream of the start codon, and a stop codon followed by a PGK-neomycin resistance cassette was placed in-frame at codon 123, inadvertently truncating the last 11 amino acids. The mutation was created in ((129X1/SvJ x 129S1/Sv)F1-Kitl+) - derived R1/E embryonic stem (ES) cells.