My favorite molecule is testosterone.
It is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is secreted primarily in the testicles of males and the ovaries of females, although small amounts are also secreted by the adrenal glands. Why is it my favorite? The reason it is my favorite structure is because it plays a key role in the development of male reproductive tissues such as the testis and prostate as well as promoting secondary sexual characteristics such as increased muscle, bone mass, and the growth of body hair. This, to me is great stuff for a single hormone. In addition, testosterone is essential for health and well-being as well as the prevention of osteoporosis. Testosterone is observed in most vertebrates. Fish make a slightly different form called 11-ketotestosterone. Its counterpart in insects is ecdyson.