Ernest Armstrong McCulloch was a medical researcher who studied the biology of blood cells and blood cancer in Toronto, Ontario, in the second half of the twentieth century, and who, with James Till, demonstrated the existence of stem cells in the blood, called hematopoietic stem cells. Stem cells are cells that can develop into more specialized cells in the body. Hematopoietic stem cells, or HSCs, are a type of stem cell that, when present in blood and bone marrow, can develop into specialized blood cells, such as red blood cells, white blood cells, and platelets. Since the start of the twentieth century, researchers hypothesized the existence of something with the self-renewal properties of what were later called stem cells but lacked evidence identifying those stem cells. McCulloch’s work provided the necessary evidence, which laid the foundation for identifying the function of stem cells in other tissues. Through his work leading to the identification of blood stem cells, McCulloch aided the development of treatments for blood cancers, which affect roughly 1.6 million people in the United States.