Diethylstilbestrol is a synthesis, non-steroidal form of estrogen hormone. It is used in broiler chicken as feed adjunct or as an animal implant to stimulate animal growth and reduces the feed costs.
In addition to that, slightly increases the moisture, protein, and ash and decreases the fat content in meat. In the U.S and other countries, it was common practice for animal meat production.
Proteolytic enzymes have injected into chicken shortly before it is slaughtered so that the pumping action of the heart circulates the tenderizer throughout the tissues.
As a result, when the chicken has slaughtered and the meat cuts are prepared, they have more tenderness.
If you interested, blog post
Tenderization of chicken and it’s side effects