The mammary stem cell field wakes up to hibernating cells
The definition of a true stem cell has long been thought to be that of a rare cell that preserves its long-term self-renewal potential and avoids compromising its genetic material by remaining inert. More recently, we have come to appreciate that different tissues harbor different stem cells that suit their requirements, including cycling and non-cycling stem cells (1). The existence of quiescent stem cells has been reported in many tissues, but until recently, such a stem cell has not been convincingly shown in the mammary gland. In a recent issue of Cell Stem Cell, Cai and colleagues  reported the identification of a quiescent stem cell that expresses high levels of Bcl11b and is required for long-term maintenance of the mammary gland (2).