An oocyte is an immature egg cell in the ovaries. Before a woman is born, her ovaries will contain about five million eggs. At birth, about three million of those egg cells die — apparently by committing suicide. This fact of nature has long puzzled scientists. Now, Harvard researchers working at the Center for Blood Research have uncovered a critical clue that may lead to a greater understanding of infertility and miscarriage. Oocytes are killed by proteins called caspases. Studies in worms have suggested that the caspases are triggered by a unique set of signals in oocytes. But up until now no one has been able to discover those molecular signals. Rosa Navarro, Keith Blackwell, and their colleagues recently identified one such signal — a defect in a protein needed for processing RNA. In worms lacking the protein, oocytes underwent mass suicide. The findings in worm oocytes could shed light on questions of human concern, such as infertility. “I think we’re plugging into something that’s involved with what makes a good oocyte,” Blackwell said. Being able to distinguish good oocytes from bad could yield information about the potential for birth defects, miscarriages, and infertility.