I wrote a small article about YK11 a while back. It looks like it could be a very good myostatin inhibitor, without all those messy nosebleeds ACE031 caused.
OK, having done a bunch of reading, this is what I understand about YK11:
YK11 stimulates the creation of follistatin. When YK11 is in the presence of follistatin, it causes a large sudden production of myogenic differentiation factor (MyoD), myogenic factor 5 (Myf5), and myogenin. The three myogenic items are what causes the body to create new muscle cells. Testosterone and DHT compounds also do this, but without the creation or need for follistatin. Well, not quite correct, testosterone does induce the creation of follistatin, just not nearly at the same level as YK11. Normally, the body produces myostatin as a counter to mygenin (it competes for the same receptors and then encourages the body to destroy the newly created muscle cells), to help keep the body from just making more and more muscle (runaway muscle creation would be terrible). Follistatin aids in the reducing the effect of myostatin in a few ways, but that is outside the scope of what I am talking about. It is this follistatin that makes YK11 so powerful compared to testosterone or DHT compounds (and one of the many reasons why testosterone is still a better choice for muscle creation than other AAS - it is a perfect item to use in synergy with other items).
The human body, being an insanely amazing thing, is not powerless against follistatin. As follistatin levels rise, it creates an follistatin antibody. This anti-body helps restore the body back to its normal state so there is no run-away muscle creation (again, that would be a bad thing).
YK11 is only partially androgenic and it has the same effects on FKBP51 as DHT compounds do, in other words, it increases it quite a bit. This is only a problem if you currently have cancer, as FKBP51 is instrumental in cell survival and it will also greatly aid cancer cells in their survival.
If you have cancer, do not take YK11...and also do not take any compound that turns into DHT in the body. Yeah, that is just about every good AAS out there. Remember, cancer is no joke, so if you have it, getting rid of it should be your number one priority...not gaining muscle. Don't be a loser about it and die just to look good at your funeral. For the vast majority of people without cancer, there is no problem with having an increased FKBP51 level.
Increased FKBP51 levels does NOT cause cancer.
The only side effect from YK11 use (other than the increase in FKBP51, which you also find in most AAS), found so far is a weakening of the connective tissues in joints, but it is still a very new SARM so only time will tell. I would highly recommend using a connective tissue strengthening substance while using YK11, such as HGH or IGF. These substances help strengthen the connective tissues, so they will offset any damage caused by YK11 There is also no suggested dosing by the medical community, again due to it being so new. As far as I can tell, it has yet to be used on a live creature by the medical community - it has only been used on cell cultures. At least they were human cell cultures. Welcome to the bleeding edge here folks! I have included a nice picture showing that follistatin and myostatin compete for the same receptors. Myostatin encourages muscle atrophy, follistatin encourages muscle creation (more or less correct for both).
YK11 also does not appear to cause the creation of Estrogen - it actually seems to do the same thing as Tamoxifen (though through a completely different method) but to a far lesser degree.
YK11 cannot be used as a SERM replacement so please do not try.
What we need to keep an eye out for is nosebleeds. ACE31 was a myostatin inhibitor with great promise, made it into phase 2 human trials where they tested it on both people with muscular dystrophy and healthy people. It caused bad nosebleeds that stopped when the drug was no longer taken. It has been shelved until someone can figure out how to stop the bleeding. Since YK11 also appears to inhibit myostatin (though through completely different mechanisms), there is always the chance it will cause nosebleeds as well. They have no idea why ACE31 causes nosebleeds, but it did it with enough power and frequency to kill what looked like a huge money maker for the big pharma developing it.
I doubt YK11 will cause nosebleeds, due to the way it works, but it is better to keep an eye out for it and it never happen than it to happen and you think it is caused by the change in seasons.
For me, YK11 looks like something I would like to try out. I think it, combined with a good AAS regimen, could help push me to greater heights.
OOPS!!! I forgot to add the sources! Unsourced info is always suspect. 
https://www.ncbi.nlm.nih.gov/pubmed/21372378
https://www.jstage.jst.go.jp/article/bpb/36/9/36_b13-00231/_html
