Researchers have rejuvenated aged hematopoietic stem cells (HSC) to be functionally younger, offering intriguing clues into how medicine might one day fend off some of the ailments of old age. HSCs are stem cells that originate in the bone marrow and generate all of the body's red and white blood cells and platelets. They are an essential support mechanism of blood cells and the immune system. As humans and other species age, HSCs become more numerous but less effective at regenerating blood cells and immune cells. This makes older people more susceptible to infections and disease, including leukemia. Researchers in the current study determined a protein that regulates cell signaling -- Cdc42 -- also controls a molecular process that causes HSCs from mice to age. Pharmacologic inhibition of Cdc42 reversed HSC aging and restored function similar to that of younger stem cells.