Study the diagram on page 239 of the lac operon.  Note that lactose is the inducer in this system.  This is a logical control mechanism.   Why would a cell make enzymes for breaking down lactose if lactose were not present in the cell? That would be a big waste of energy! Also, note that the text refers to structural genes (term used in Study Guide as coding regions). Be sure you can explain the function of the regulatory gene, the promoter, operator, and structural genes or coding regions.