Ligand trap.

An engineered molecule that binds a signalling protein before it reaches its receptor.

A ligand trap is an engineered molecule designed to bind a circulating signalling protein and sequester it, preventing it from reaching its receptor. It is a mechanistic alternative to blocking the receptor itself: instead of occupying the lock, the trap removes the keys. Traps are commonly built as decoy receptors, using the binding portion of the natural receptor fused to an antibody fragment to keep it in circulation, and they are typically proteins rather than short peptides. They appear in this glossary because they are the dominant approach in areas that peptide discussion touches, particularly myostatin and activin signalling, where the goal is to reduce a growth-limiting signal without blocking a receptor that other important ligands also use. The distinctive strengths and weaknesses follow from the mechanism. A trap acts on whatever is circulating, so it is well suited to a soluble signal but does not help against a signal acting locally within a tissue. And selectivity is a genuine challenge, because related family members often share binding surfaces, so a trap intended for one ligand frequently captures its relatives too, which is a common source of unintended effects in this class.