A targeting vector containing point mutations D303A, D357A, and D359A in exon 13 as well as a floxed neomycin gene in intron 12 was electroporated into (129X1/SvJ x 129S1/Sv)F1-Kitl+-derived R1 embryonic stem (ES) cells. The neomycin gene was removed by crossing with a Cre deleter strain, which left the expression level at 20-30% of wild-type. (J:118319, J:131423)