Myostatin.

A protein that limits skeletal muscle growth.

Myostatin, also called GDF-8, is a member of the TGF-beta family produced by muscle cells that acts as a brake on muscle growth. It signals through activin receptors to restrain the proliferation and differentiation of muscle precursor cells, keeping muscle mass within a set range. Its role became widely known through naturally occurring loss-of-function variants: cattle breeds with striking muscle development, and rare documented cases in other species and in humans, all carry disruptions in myostatin signalling. That makes it a genuinely compelling target, and blocking myostatin or its receptors has been pursued as a therapeutic strategy for muscle-wasting conditions, using antibodies, receptor blockers, and ligand traps rather than peptides in most cases. Two points deserve emphasis. Myostatin's brake is set during development, so removing it in an adult is not the same experiment as never having had it, and the results in adults have generally been more modest than the dramatic animal examples suggest. And the pathway is not muscle-specific, since the same receptor family carries other signals, which is a source of off-target concern. Compounds marketed directly to consumers as myostatin inhibitors generally have no credible human evidence of altering myostatin signalling at all.