Well, your immune system is already capable of identifying and deleting cells that have escaped the normal cell cycle control and have become cancerous, but if

Well, your immune system is already capable of identifying and deleting cells that have escaped the normal cell cycle control and have become cancerous, but if there's damage to your immune system then it can lose that capability. Immune system damage can occur by many means: viral infections, chemical carcinogens, ionizing radiation, etc. Using gene editing technology to repair your immune system using a human template wouldn't be transgenic technology and is probably a relatively safe use of CRISPR gene editing, for example. Throwing in some shark genes to see what happened, because some species of sharks don't seem to get cancer IIRC, that would not be such a great idea I don't think.