Targeting methods replacement vector

Homologous recombination occurs approximately 1000-fold less frequently than non-homologous recombination, therefore methods have been developed to enrich for homologous recombination events. A widely applied method includes the use of a positive and a negative selection marker and does not require the expression of the target gene in ES cells (Mansour et al., 1988). The targeting construct is based on a replacement-type vector... 

---