David Herman MacLennan. 3 July 1937—24 June 2020

David Herman MacLennan, one of Canada's foremost biomedical scientists, was known internationally for his research on the molecular mechanism of muscle contraction in human health and disease. David was born on 3 July 1937 in Swan River, Manitoba, and grew up in farm country. After obtaining a BS (Agriculture) in plant science from the University of Manitoba in 1959, David completed his MSc (1961) and PhD (1963) in biology at Purdue. A post-doctoral fellowship at the Enzyme Institute at the University of Wisconsin followed, where he was appointed as an assistant professor (1964–1968). At Wisconsin David published a series of elegant papers on the isolation and characterization of the mitochondrial ATPase and protein components of the electron transfer system. In 1969 he was recruited back to Canada as an associate professor in the Banting and Best Department of Medical Research at the University of Toronto, where he spent the rest of his illustrious career. Here, David shifted his focus to determine how calcium regulates muscle contraction, with a focus on the role of the sarcoplasmic reticulum (SR). David was a scientist who knew where a field was going and he often got there first, incorporating new technologies along the way. His early discovery of the Ca 2+ ATPase pump that controls calcium uptake into the SR was the key to muscle relaxation. His lab systematically characterized the components of the SR, including the ryanodine receptor which acts as a calcium release channel to allow muscle contraction. David's discoveries of these molecular mechanisms and their application to debilitating muscle disease are an inspiring scientific legacy. Although David published hundreds of papers, many cited hundreds of times, gave hundreds of invited seminars and won many prestigious awards, including Fellow of the Royal Society of London in 1994, his greatest legacy is the people he trained, many of whom went on to leadership positions in research and at universities around the world.