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Most people who die from cancer are killed by cells that have spread through their body, but we know relatively little about how they spread. Now a team has genetically sequenced the secondary tumours of 10 women who died from breast cancer, and found that there are usually just two or three waves of migration from the original tumour.

Carlos Caldas of the Cancer Research UK Cambridge Institute and his colleagues genetically sequenced samples from an average of 19 secondary growths per person. Some of the women, however, had hundreds of secondary tumours.

Because all the tumour cells in a person’s body descend from a single cell with cancerous mutations, they were able to draw a “family tree” for each woman, showing how the tumour cells were related, and revealing how long ago they split from each other. “The number of mutations is effectively a clock,” says Caldas.

The patterns of mutation suggested that the maximum number of spreading events any woman had was three, while the lowest was one. “There was a very limited number of these founding events, which must have been a burst of cells into the circulation,” says Caldas.

The finding will help us to “know our enemy”, he says. Other teams are developing ways of catching and removing cancer cells as they spread through the blood.