Fragment.

A portion of a larger peptide or protein.

A peptide fragment is a sub-sequence taken from a larger peptide or protein. Fragments are studied to isolate which part of a molecule is responsible for a given activity, on the reasoning that if a short stretch of the sequence carries the effect, that stretch may be easier and cheaper to make than the whole molecule and may avoid some of the parent's other actions. Several widely discussed research peptides are described this way, including fragments derived from growth hormone and from proteins involved in tissue repair. The concept is scientifically real and it is how a great deal of early drug discovery works. The important caution is that a fragment is a genuinely different molecule from its parent, not a smaller dose of it. Cutting a sequence changes its shape, its stability, its receptor binding, and often its effect entirely, and a fragment can lose the parent's activity, keep only part of it, or do something unrelated. The common marketing move worth recognising is describing a fragment as though it inherits the reputation of the well-studied parent molecule. Evidence for the parent hormone is not evidence for the fragment, and each fragment has to be evaluated on its own research record.