the cell's contents. He then filtered out the cell membranes and placed the remaining cell contents in a centrifuge to separate them according to mass. He divided the centrifuged contents into fractions, each of a specific mass, and discovered that particular fractions were responsible for particular cell functions. In 1938 he identified and purified for the first time component of Rous sarcoma virus, the causal agent of carcinoma, as "ribose nucleoprotein" (eventually named RNA). He was the first to use electron microscope to study biological cells. Earlier electron microscopes were used only in physical researches. His first electron microscopic study was on the structure of mitochondria in 1945. He was given American citizenship in 1941. He discovered that mitochondria are the "power houses" of all cells. He also discovered cytoplasmic granules full of RNA and named them "microsomes", which were later renamed ribosomes, the protein synthesizing machineries of cell. With his associate, Keith Porter, he found a "lace-work" structure that was eventually proven to be the major structural feature of the interior of all eukaryotic cells. This was the discovery of endoplasmic reticulum (a Latin for "fishnet"). In 1949, he became Director of the Jules Bordet Institute for Cancer Research and Treatment (Institut Jules Bordet) and Professor at the Faculty of Medicine of the Free University of Brussels, where he was Emeritus in 1971. In the mid sixties during an Electron Microscopy symposium in (Bratislava)-(Czechoslovakia) organized by the (UNESCO) at the Czechoslovak Academy of Sciences, he meets young scientist Dr. Emil Mrena who was at that time head of the Electron Microscopy department. He invited him to come and work with him in Brussels, making it possible for Dr. Mrena's family to escape the communist regime. Their close collaboration gave fruition to 5 publications from 1969 to 1974. With the support of his colleague and friend Christian de Duve, he became in 1972 Professor at the University of Louvain (Université catholique de Louvain) and Director of the "Laboratoire de Biologie Cellulaire et Cancérologie" in Louvain-la-Neuve where he moved with Dr. Emil Mrena as sole collaborator. At the same time, he was appointed Professor at the Rockefeller University, an institution with which he had remained connected, in different degrees, since 1929. Personal life He married Julia Gilder in 1935, with whom he had a daughter, Philippa. They were divorced while he was at Rockefeller. Philippa became a neuroscientist and married Antony