A targeting vector was generated by flanking exons 5 and 6 with a 5' loxP site and a 3' Frt-PGK-neomycin-Frt-LoxP expression cassette. Germline transmitting mice were bred sequentially with CAGG-Flpe mice to remove the neo cassette, and then to EIIa-Cre transgenic mice in FVB background to generate the null allele. (J:137185)