Tumor suppressor genes control protooncogenes.
You've probably been reading a lot about p53 in the newspapers. It is supposed to stop these cells from dividing until the damaged DNA can be repaired. These damaged cells can be cancerous, so that's one reason why the gene is so important. Some people have mutations of this tumor suppressing gene, and it has been implicated in many different forms of cancer. So, we've got two different things we're looking at here.