Mildred Cohn, who pioneered using stable isotopes to trace how enzymes work in cells, has died, reports the Los Angeles Times. She was 96. In addition to her work using stable isotopes to study biochemical reactions, Cohn also used NMR to study the role of ATP as an energy source in the cell. Over the course of her career, Cohn worked with many Nobel laureates, including Harold Urey, as she struggled with prejudice.