Yoshio Masui, whose curiosity about how cells proliferate led to a new understanding of the mechanisms behind cell division, has passed away at the age of 93. In 1961, having recently earned his doctoral degree, Masui opened a lab at Konan University in Japan where he began observing the activity of maturation in frog eggs at various stages of development. Masui later moved to Yale University, and eventually to the University of Toronto, where he continued his exploration of the development of fertilized frog eggs—a model organism he selected for its simplicity.

Masui knew that egg growth could be stimulated by the hormone progesterone, but it only worked when the surface of the eggs were exposed to progesterone—not when the hormone was injected directly into the oocytes. He concluded that progesterone acting on the egg’s surface must affect something in the cytoplasm of the cell that, in turn, stimulates cell division. He set out to find that “something,” which turned out to be an activity in the cytoplasm that he called MPF—for maturation promoting factor.

Masui’s newly-discovered promoting factor was later found to be analogous to similar promoting factors discovered in budding yeast (by Lee Hartwell) and in fission yeast (by Paul Nurse). The discoveries of key proteins necessary for cells to proliferate revealed the universal machinery for regulating cell division in all eukaryotic organisms—and earned Masui the 1998 Albert Lasker Basic Medical Research Award (an honor he shared with Nurse and Hartwell).