Rapamycin is a widely used drug for cancer and transplant patients, and there are hints that it might even help us put off old age and live longer. But, it also comes with a downside: rapamycin leads to diabetes in as many as 15 percent of the people who take it. Researchers at Dana-Farber Cancer Institute have figured out that the drug turns the insulin signal off in muscle, preventing muscle cells from taking blood sugar in. The team found that the key is a single transcription factor in muscle known as YY1. Mice lacking YY1 are protected from diabetes when they take rapamycin.That's because rapamycin changes YY1 within muscle and that shift leads to chemical and structural modifications of the genes encoding insulin signals. Those changes effectively turn insulin off, keeping sugar from coming in. The findings support the notion that at least some people taking rapamycin should also take anti-diabetic drugs.