PNA probes normally form double helices, unless the PNA probe contains a long pyrimidine stretch. For PNA probes that are homopyrimidine (5 or more pyrimidines in length) or contain 7 or more pyrimidine nucleobases in a row hybridization may produce a triple helix. It is also important to note that PNA probes can hybridize in both the parallel and anti-parallel orientations, unlike DNA oligomers that will only hybridize in the anti-parallel orientation.