I saw this last night and naively thought this would be a decently covered story today:

The ability to recreate breast cancer in the laboratory, understand why the cancer can recur in treated women and even grow replacement breast tissue could result from the discovery of a “mammary gland stem cell” reported today by scientists.
Stem cells are the parents of other cell types in the body. Rare mammary stem cells, isolated in mice, are the originators of breast tissue.
Remarkably, two teams report today that it is possible to reconstruct an entire organ from a stem cell, which in this case is the branched ducts and structures made of so called epithelial tissue in a milk producing breast.
The discovery lays an important foundation for understanding how normal breast tissue develops. It could also provide clues about how breast cancer occurs and how rogue tumour cells can evade treatment to cause a recurrence.