MK-677 which is also known as Ibutamoren or Nutrobal is a Selective Androgen Receptor Modulator (SARM). Most newer SARMs are non-steroidal but have similar anabolic properties to steroids. MK-677 and most SARMs differ from steroids by how they bind to androgen receptors (ARs). Most of the popular steroids are full agonists of the ARs however, non-steroidal SARMs selectively bind to specific ARs due to their unique chemical composition.The majority of SARMs developed target primarily lean muscle mass and bone tissues. It has a high affinity for ARs in lean muscle and bone tissue and a lower affinity in other tissues.