Hematopoietic stem cells—bone marrow-derived adult stem cells that give rise to the wide variety of specialized blood cells—come in two flavors: the reserve force sits quietly waiting to be called upon while the active arm continually proliferates spawning billions of blood cells every day. In their latest study, researchers at the "http://www.stowers.org/" Stowers Institute for Medical Research, with the participation of Rebeca Sánchez, from the Cell and Gene Therapy group at Vall d’Hebron Institute of Research, reveal a new mechanism that is critical in maintaining the delicate balance between the two. Publishing in the last issue of Nature, the team led by Stowers Investigator Linheng Li, Ph.D. reports that genomic imprinting, a process that specifically shuts down one of the two gene copies found in each mammalian cell, prevents the reservists from being called up prematurely. “Active HSCs (hematopoietic stem cells) form the daily supply line that continually replenishes worn-out blood and immune cells while the reserve pool serves as a backup system that replaces damaged active HSCs and steps in during times of increased need,” explains Li. “In order to maintain a long-term strategic reserve of hematopoietic stem cells that lasts a lifetime it is very important to ensure that the back-up crew isn’t mobilized all at once. Genomic imprinting provides an additional layer of regulation that does just that.”