Figure 1.

The highway code of mammary gland biology. A schematic representation of the processes involved are depicted in the top: stem cells (red) divide symmetrically (giving rise to two stem cells) or asymmetrically (generating one stem cell and one progenitor cell) [11]. Progenitor cells proliferate (green; progenitor cell expansion) before initiating their maturation to differentiated cells (blue; progenitor differentiation). These processes may be accelerated (represented by green traffic lights), stopped (red traffic lights), or orientated through specific directions (direction arrows) under the regulation of driver genes, either Bmi1, genes up-regulated by pregnancy, or the Notch signalling pathway.