TB-500 is a synthetic fragment of thymosin beta-4, a naturally occurring protein involved in cell movement and tissue repair. The fragment corresponds to the region of the parent molecule that binds actin, the protein that forms part of a cell's internal scaffolding. The reasoning behind it is that if this small section does the useful work, you can synthesise just that section. Whether that reasoning holds has never been tested in people. Every study cited in marketing was performed on the full-length protein, which is a different molecule with a different development history and its own separate trial record. TB-500 itself has no completed human trials of any design. The distinction between the fragment and the parent is the single most important thing to understand about it, and it is the distinction almost every vendor page collapses.