Answer the following question given this paragraph: 

Hormones are the messenger molecules of the endocrine system. Endocrine hormones travel throughout the body in the blood. However, each hormone affects only certain cells, called target cells. A target cell is the type of cell on which a hormone has an effect. A target cell is affected by a particular hormone because it has receptor proteins that are specific to that hormone. A hormone travels through the bloodstream until it finds a target cell with a matching receptor it can bind to. When the hormone binds to a receptor, it causes a change within the cell. Exactly how this works depends on whether the hormone is a steroid hormone or a non-steroid hormone . At the link below, you can watch an animation that shows how both types of hormones work. http://www. wisc-online. com/objects/ViewObject. aspx?ID=AP13704.


Q: What are the messenger molecules of the endocrine system?


A:
hormones