Canadian scientists have made a breakthrough in medical science, becoming the first to decode the DNA of breast cancer cells.

The British Columbia Cancer Agency study represents a major step towards determining how cancer begins and what makes it spread through the body.

Researchers have decoded all three billion letters in the DNA sequence of a spreading breast cancer tumour.

They have also identified the mutations that caused the original cancer to spread.

Until now scientists have not been able to to find all the mutations present in a given tumour.

Medical experts say the finding brings unprecedented information about breast cancer; there were genetic distinctions between an original tumour and after it spread, and that could lead to new cancer drugs and more personalised medicine.