Primer

A short piece of single stranded DNA needed for detecting corresponding DNA seqences by PCR

Primers are the critical ingredient for detecting specific DNA sequences using PCR (polymerase chain reaction). Custom designed primers can be used to find out if a DNA sample contains DNA from a GMO.

The primer is a mirror copy of a piece of a DNA sequence of interest. It usually consists of 18-24 base pairs. 

PCR detection depends on primers matching up to the DNA sequence of interest. If present, the association sets off a chain reaction that copies the sequence of interest millions of times. It is then fairly easy to find out if the sequence of interest is present.