YK11 (Myostatin Inhibitor / Steroidal SARM) is a synthetic steroidal selective androgen receptor modulator (SARM) that distinguishes itself from other members of the class through its dual mode of action. Unlike typical SARMs that primarily exert anabolic effects via selective androgen receptor binding, YK11 has been shown to induce the expression of follistatin, a glycoprotein that functions as a potent myostatin antagonist. By antagonizing myostatin (a negative regulator of skeletal muscle growth), YK11 potentially unlocks supraphysiological muscle hypertrophy beyond the limits imposed by endogenous regulatory pathways. Its anabolic activity is mediated through partial agonism of androgen receptors, leading to increased transcription of target genes involved in muscle differentiation and repair, while the follistatin–myostatin axis provides an additional layer of growth-promoting influence. Preclinical studies suggest YK11 may influence key signaling cascades such as the Smad pathway, which is central to myostatin’s inhibitory effects on muscle cells. By disrupting Smad-dependent transcription, YK11 enhances myogenic differentiation and proliferation of muscle satellite cells, thereby augmenting tissue regeneration and anabolic signaling. Although the pharmacokinetic and toxicological profile of YK11 has not been fully elucidated in humans, in vitro research indicates a unique convergence of SARM-like anabolic signaling with myostatin suppression, positioning YK11 as a hybrid compound of interest in the exploration of muscle growth, repair, and metabolic regulation.