Метиловый эфир (17a,20E)-17,20-[(1-метоксиэтилиден)бис(окси)]-3-оксо-19-норпрегна-4,20-диен-21-карбоновой кислоты

YK-11 is a synthetic steroidal selective androgen receptor modulator (steroidal SARM). It is a steroid compound based on 5-α-dihydrotestosterone (DHT), which is a gene-selective and partial agonist of the androgen receptor (AR). YK11 is so far the only SARM that is able to increase the expression of the activin binding protein Follistatin (FST), and right the induction of FST plays a key role in its anabolic effects. YK11 has chemical formula C25H34O6 and molecular mass 430.541 g·mol?1.

YK11 is a steroidal SARM (Selective Androgen Receptor Modulator). More specifically, it’s a Myostatin inhibitor. It’s considered to be one of the most powerful SARMs on the market and it’s actually comparable to certain steroids in its effectiveness.
YK11 is also a myostatin inhibitor (myostatin reduces our muscle mass), which means that you won’t be losing muscle while cutting and will very likely gain some instead, making it a great compound for cutting, lean bulking and especially body recomposition.

Well there are a few obvious benefits to taking a daily supplement designed to help build muscle.
First and foremost, YK-11 helps you grow more muscle in a smaller amount of time. YK-11 is a myostatin inhibitor which makes it easier for your body to develop more muscle.
In addition to more bulk, your muscles will become stronger from the production of follistatin. So as you can imagine, the stronger your muscles are the more you can do with them. Like steroids, YK-11 and other SARMs promote bone health and can make your bones stronger.
As mentioned earlier, YK-11 also helps with fat loss by replacing that fat with muscle.There are also claims that YK-11 helps boost libido and that it allows you to work out harder and longer. YK-11 can also help you retain the muscle that you have built.

Possible side effects of YK11 have not yet been scientifically investigated (at all). Theoretically, also the following side effects are not excluded:
Suppressed levels of natural Testosterone in the body (quite likely)
Increased risk of heart attack and stroke
Liver toxicity (due to the structure derived from DHT)
Can increase the progress of some cancer types (due to the increased expression of Follistatin)
The bodybuilding community on the Internet has experienced and reported with respect to YK11 some typical side effects, that occur with steroid use:
Mild testosterone suppression
Fatigue, low energy level
Aggression
Joint pain
Excessive hair loss
Mild acne

[1] Tomofumi Yatsu. “Selective Androgen Receptor Modulator, YK11, Up-Regulates Osteoblastic Proliferation and Differentiation in MC3T3-E1 Cells.” Biological pharmaceutical bulletin 41 3 (2018): 394–398.