Ernest McCulloch, the Ontario Cancer Institute cell biologist who co-authored the first evidence demonstrating the existence of stem cells, died on Wednesday at the age of 84.

Together with his colleague James Till, McCulloch published a landmark paper in 1963 that defined the hallmark properties of stem cells — namely, the ability to self-renew and differentiate into other tissue. A lifelong scientist, McCulloch maintained an active research program studying the role of stem cells in acute myeloid leukemia, and even published a paper just last month on the radiation sensitivity of mouse bone marrow cells.

McCulloch was “a giant in Canadian health research,” Alain Beaudet, president of the Canadian Institutes of Health Research, told Nature Medicine in an email. McCulloch and Till’s “work provided the theoretical underpinning for bone marrow transplantation and advanced medical treatment of leukemia and autoimmune diseases. Their work also laid the foundation for the promising field of regenerative medicine.”

In 2005, McCulloch and Till took home the Lasker Award for Basic Medical Research — an accolade that prompted the two scientists to write this commentary in Nature Medicine reflecting back on their seminal research into the basic properties of stem cells.