Correctly targeted ES cells were microinjected into blastocysts to produce chimeric mice. Chimerics were bred to C57BL/6J mice. Following germline transmission of the targeted allele, heterozygous mice were interbred to produce wild type (+/+), heterozygous (+/-) and homozygous (-/-) animals. Mice were genotyped by Southern blot analysis. The Exon 6 specific probe hybridized to only a ~16 kb BglI restriction fragment in +/+ mice, a ~11 kb BglI fragment in -/- mice, and to both of these fragments in +/- mice (Fig. 1B).