You use a viral vector to deliver the DREADD gene to whatever cells you want to control. Once the DREADD is expressed on the cell surface, it just sits there, inert. Then you administer CNO, which binds exclusively to the DREADDs. The receptor didn't exist in your body before. Its binding partner (CNO) doesn't exist endogenously. As a result, you, and you alone, get to decide when and where it is turned on.
