Figure 1. Recombineering strategy for allelic exchange using positive selection.

After transformation of a vector containing flanking homology arms to direct recombination to a particular chromosomal locus, a first crossover event leads to integration of the vector into the chromosome. The vector recombines out through a second crossover event, producing a mutant allele or regenerating the wildtype allele depending on whether the upstream or downstream homology region is used for recombination. pKFC-mediated deletion of chromosomal genes utilizes this recombineering strategy.