Blood contains red blood cells as well as several types of white blood cell. As cells wear out with age, they are constantly replaced throughout a person’s lifetime. Stem cells that have the potential to become every type of blood cell are the source of new blood cells. How do stem cells know what type of blood cell to become? This depends on which genes are switched on in the stem cells. Meinrad has discovered that Pax5, a protein involved in switching genes on, directs stem cells to turn into a type of white blood cell called B cells.