YK-11 is a synthetic, steroidal selective androgen receptor modulator (SARM) structurally derived from dihydrotestosterone (DHT). It is currently classified as an experimental compound and has not been approved for medical use in humans. Despite this, it has been widely researched in preclinical settings for its potent anabolic effects on muscle cells. YK-11 functions as a partial agonist of the androgen receptor. It activates the receptor but not to the full extent like natural androgens such as testosterone or DHT. It does not promote the typical N/C-terminal interaction within the AR, which is required for full transcriptional activation. Instead, YK-11 induces muscle growth through a secondary mechanism: it significantly increases the expression of follistatin, a potent inhibitor of myostatin—a protein that suppresses muscle growth.