### GC clamp

The term GC clamp refers to the presence of up to three guanines and cytosines (GCs) at the 3' end of a primer. Some tools require the GCs to occur directly at the 3' end, while others use a broader definition, for example, requiring two GCs in the last five bases. We use the more narrow definition requiring the GCs to occur terminally.

Primers with GC clamps are favorable because polymerase synthesizes new DNA from the 5' to the 3' end and attaches to the 3' end of primers. Since GCs provide stronger binding affinities than the other nucleotides, the GC clamp is supposed to stabilize the binding of the polymerase and prevent it from breaking away.