A fragment is a continuous stretch of a larger parent protein, identified by the residue numbers it spans. The notation is literal: 176-191 means residues 176 through 191 of the parent numbering, counting from the mature protein’s N-terminus. HGH Fragment 176-191 is therefore a 16-residue piece of the C-terminal region of 191-residue human growth hormone.
Why the fragment concept matters in peptide research
The central point is that a fragment is not a scaled-down parent. Growth hormone acts through a receptor that requires dimerisation across a large interface, and a 16-residue piece cannot reproduce that; published work on the 176-191 region examined lipolytic activity attributed to that domain specifically, not growth-hormone signalling. The same logic applies to TB-500, an actin-binding fragment of the 43-residue thymosin beta-4 protein, and to IGF-1 DES, a truncated variant of IGF-1.
Two practical consequences. Numbering conventions differ between precursor and mature protein, so a fragment name is ambiguous unless the reference sequence is stated. And results published for a parent protein should never be transferred onto its fragment. See HGH Fragment 176-191 explained and why a fragment is not the whole protein.