Adenine
From Memory Alpha, the free Star Trek reference.
Adenine is a chemical compound found in Human DNA. It is always paired with thymine. Adenine is represented in the genetic code with the letter "A." (TNG: "Unnatural Selection")
Adenine, along with radodine, lidestolinine, and asporanine, were the ingredients to the cure for the morphogenic virus introduced to the Changelings by Section 31. (DS9: "Extreme Measures")
