MSTN

In one line: The body's "stop building muscle" brake — blocking it lets muscles grow bigger.

MSTN (myostatin) is a protein your body uses to cap how much muscle you can build — a built-in brake so muscles don't grow without limit. Animals (and rare humans) with a broken myostatin gene are extraordinarily muscular. Experimental "myostatin inhibitors" (and the compound follistatin, which neutralises it) aim to release this brake.

The promise is obvious; the reality is that human safety is unknown and early drugs in this class were stopped for side effects. A compelling idea that isn't proven or safe yet.