TB-500 differs from the whole protein it copies.

What the name points to

A lab makes TB-500 as a chain of seven amino acids, the linked units used to build proteins. It copies part of thymosin beta-4, which has forty-three of those units.

The short lab-made piece, or fragment, also has a small added chemical part at one end. Scientists call this addition a cap. It is part of the substance itself, not a bottle lid. Only the longer protein occurs naturally in the body; TB-500 is not a substance the body makes.

Why a near match is not enough

The FDA says that adding this extra part can change how the short piece behaves. Studies of a piece without that addition cannot tell us how TB-500, which has the addition, will act.

This is not a spelling detail. The longer protein and the short fragment are not interchangeable names for the same thing.

Ask for more than a familiar label

The FDA also found that the name TB-500 was being used for more than one chemical version of the substance. Its review calls the naming confusion a safety concern because someone could receive a different substance from the one intended.

Ask for the substance’s full name in writing, including which version it is and how that version differs from others. Then ask whether the research supplied uses that same substance. If the answer changes between the offer and the paper, ask for an explanation of the change.