Describe the role of signal molecules in activating gene expression at the appropriate time

Some genes only need to be expressed or produced when needed. For example, fibrinogen is a protein that needs to be produced when a person needs to clot. A signal protein 'signals' or tells the DNA to start producing that particular protein. Imagine if the protein was produced all of the time at the same quantities ... a person would be prone to strokes and other sicknesses that are caused by blocking the flow of blood to tissue.