**4.5.1 PCR based approach**

A convenient method for generating linear targeting vectors is via a PCR based approach in which primers are designed against two important sequence elements. The 5' end of the primer contains sequence homology to the targeted locus while the 3' end contains sequence for PCR amplification of the targeting construct. When these primers are used in a PCR reaction, the product produced contains both homology to the target and the targeting construct. Using a PCR based approach allows an investigator to use the same template to target an alternative locus. By changing the 5' homology of the primers to match an alternative locus, a new linear targeting construct can be generated. Therefore, the template used for generating the linear targeting construct is modular.
