A targeting vector was used to place a floxed neomycin-thymidine kinase expression cassette downstream of exons 2, 3, and 4, and a loxP site upstream of the exons. The construct was electroporated into (129X1/SvJ x 129S1/Sv)F1-Kitl+-derived R1 embryonic stem (ES) cells. Correctly targeted ES cells were transfected with Cre expression plasmid to excise the neo fragment and leave the floxed exons. (J:101977, J:212313)