A 2.5 kb fragment of exon 3 and part of intron 3 and a 5.2 kb containing exon 5, intron 5 and exon 6 were cloned into XhoI and salI sites of target vector pKO (Lexicon). PGK promoter-derived Neomycin phosphotransferase cDNA (positive selection) and Diphtheria toxin A china cDNA (negative selection) were cloned into AscI and RsrII sites of the vector. (J:177612)