It has long been suspected that only a minority of cancer cells can seed new tumours, and now the theory has finally been confirmed. The discovery promises to open up new avenues for developing more effective cancer therapies.

In 1997, it was found that only a small subset of cancer cells taken from patients with acute leukaemia can cause the disease when injected into mice. Intriguingly, these cells, identified by Dominique Bonnet and John Dick at the University of Toronto in Canada, were similar to the stem cells found in bone marrow that mature into blood and immune cells. But no such cells had ever been discovered in solid tumours such as breast cancer.

Now Michael Clarke and his colleagues at the University of Michigan Medical School in Ann Arbor have found them. The team started by implanting tiny pieces of breast cancer tissue from nine women into the breasts of female mice with weakened immune systems. All the mice eventually developed tumours.
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