A study explains how age reshapes the blood system. In both humans and mice, a few stem cells out-compete their neighbors and gradually take over blood production. The loss of diversity results in a blood system that has a preference for producing myeloid cells, immune cells linked to chronic inflammation which underlies many different diseases. Using a new technique, researchers tracked naturally-occurring ‘barcodes’ in blood cells which can lead to new strategies that spot early warning signs of unhealthy aging long before symptoms appear, helping prevent cancer or heart disease. The technique also opens the door to studying the viability of rejuvenation therapies in humans, efforts which have traditionally been the focus of animal research.