Recurring breast cancer could be caused by newly discovered rare stem cells transformed into a "tumour factory" by genetic errors, scientists have said.
Breast cancer is the most common form of the disease in the UK
Researchers believe that as well as driving all breast development, the cells may have a key role in cancer.
The Australian team transplanted the cells into mice tissue and were able to grow a fully-functioning breast.
Writing in Nature, they say they also found the key stem cells were more numerous in pre-cancerous tissue.
The research holds out hope on two fronts: the possible development of new cancer therapies, and of new techniques for breast reconstruction.
Scientists from the Walter and Eliza Hall Institute of Medical Research in Parkville, Australia, isolated the stem cells from a structure in female mice called the breast pad.
They transplanted one of the cells into the mammary fat pad of a living female mouse from which all breast tissue had been removed.
The cell divided and eventually gave rise to all the normal types of cell found in the mouse breast to such an extent that the gland produced milk in the normal fashion.
Under normal circumstances the stem cell would produce healthy tissue.
But it is thought that genetic errors, perhaps combined with external influences, could cause the stem cell or a "daughter" cell to create faulty cells.
Although there is no proof the stem cells exist in humans, the team is almost certain they do because of the expansion of breasts during pregnancy and puberty.
The team also said that chemotherapy designed to target tumours may not be able to wipe out any of the defective stem cells in the breast as they divide at a different rate.
A spokesman for the Australian-led team said: "Chemotherapy works by targeting cells that are dividing rapidly, which is typical behaviour of cancer cells.
"But an errant stem-like cell may be more resistant to chemotherapy because it divides more slowly.
"So while chemotherapy can eliminate the bulk of cancer cells, the tumour factory itself - a breast cancer stem cell - may survive months or years later."
The team believes any wonder-drug that can switch off the faulty stem cells would be the ultimate weapon in the fight against breast cancer.
However, such a weapon is far off, as before such a drug can be made, the exact make-up of genes in normal and rogue cells need to be determined.
A spokeswoman for breast cancer support charity BreastCancer Care said the research was interesting.
"However it is still at very early stages as it has only been carried out in mice.
"Further work needs to be undertaken to determine if the same stem cells exist in humans, as well as a greater understanding of other factors that influence a person's predisposition to developing breast cancer," she added.