Southern blots of DNA derived from targeted ES cells and tails of gene-targeted mice. (A) Southern blot of genomic DNA from ES cells before electroporation (TM^wt/wt), with homologous recombination of the targeting vector (TM^wt/cyt)), and after successful cre-recombinase excision of the neo-tk genes from the targeted allele (TM^wt/cyt-cre). (B) Southern blot of tail DNA from gene-targeted mice. A single 5.4-kb band is detected in the TM^cyt/cyt mice, reflecting homologous recombination of both TM alleles with the mutant form. In (A) and (B), DNA was digested with EcoRI/Xho I and bands were detected with probe E (see Fig 1).