A hairpin loop is a loop region of unpaired bases with only one adjoining stack.
Hairpin loops are described in terms of the number of unpaired bases in the loop. A prefix (tri-, tetra-, penta-, hexa-, hepta-, octa-, etc...) and the word "loop" can be combined to describe any given hairpin. Some scientific literature also uses the term 'apical loop' or 'apical loop stem' for the hairpin loop with related stem. Alternatively, one can describe the loop as an x-nucleotide hairpin, where x is the number of unpaired bases in the loop. The structure at right is an octaloop, or an 8-nucleotide hairpin loop.
==Hairpin Loop Energy==
The energy calculation for hairpin loops depends on the size. Currently, the energy of triloops depends only on the identity of the closing base pair. Tetraloops have special energy parameters that can be found here. The energy of hairpins larger than 4 nucleotides is determined by loop size, closing base pair, and terminal mismatch.