PCR can be used to identify rare DNA sequences in DNA libraries by increasing the abundance of a particular sequence.  This is accomplished by subdividing the original library into pools of decreased complexity and screening each pool or group of pools for a given DNA sequence.  A pool that contains the desired clone is subsequently subdivided into smaller pools, each of which is screened using the same PCR method or protocol that was used for the primary screen.

Protocol: PCR-Based Screening of DNA Libraries Protocol