Several years ago, biologist Thea Tlsty’s team at the University of California, San Francisco, was studying wound cells in breast tissue. These are adult cells that divide furiously in response to injury, helping to replace those that were damaged. The typical wound cell in the breast has the ability to turn into different kinds of breast cells, each specialized for a different role in the tissue, such as producing milk. But Tlsty’s team stumbled on a subset of repair cells that could do much, much more.

This tiny subset, making up just 1 out of every 10,000 cells in the breast, are pluripotent, meaning that they can be chemically coaxed to turn into a wide range of other cells. “Cells