High Quality Content by WIKIPEDIA articles! The Overlap extension polymerase chain reaction (or OE-PCR) is a variant of PCR which can a) insert mutations at specific points in a sequence, further than ~55 nucleotides from either end and/or b) produce polynucleotides from smaller fragments. To insert a mutation into a sequence, a primer is generated depending on requirements. A single substitution may be included or a totally new sequence added at the 5' end. If a deletion is required, then the continuation of the sequence 5' of the deletion effectively becomes an addition. The 3' end must be complementary to the template strand so that DNA polymerase can copy the template from the primer (DNA polymerase can only copy DNA in one direction; 5' to 3').