Killing cancer cells is easy. Killing cancer cells without also destroying everything else is a very hard problem to solve. If this protein can force cancer cells back into healthy cells (or at least self-destruct) WITHOUT negatively affecting healthy tissues then this would be significant.
Exactly! This is precisely why this is not a small step, and hopefully will lead to similar research and treatments on other cancers.
Another way to put the significance of this is a magic chemical or drug that turns zombies back into normal people. (Ok, cancer is the opposite of a zombie, but you get the point.)