The twitch of a butterfly’s wing can initiate a cascade of increasingly substantial events, culminating in changes that are both profound and far-reaching. That’s the gist of any butterfly effect, whether it’s the formation of hurricane, the climax of a Hollywood movie, or the differentiation of a stem cell. A stem cell’s fate, it turns out, can hinge on a single protein, one that stays in telomeres but accounts for genome-wide changes. The protein, telomer repeat binding fac