CHICAGO (Reuters) – Tiny sacs released from tumour cells and circulating in the blood carry genetic information about the tumour, offering a new way to track and treat the cancer, US researchers said this week. "They contain a little piece of the tumour cell in the blood stream. If you just look at these packets, you basically know what kind of mutations are in the tumour cell," said Xandra Breakefield of Massachusetts General Hospital in Boston, whose study appears in the journal Nature Cell Biology.