Female zebrafish (Danio rerio) have an unusual tendency: if their egg cells are damaged, they can turn into males. Bruce Draper, a professor in the Department of Molecular and Cellular Biology (MCB) and Florence Marlow, a professor at the Icahn School of Medicine at Mount Sinai, have discovered that immune cells called macrophages play a key role in this process. These cells normally keep things “tidy” by removing dead or damaged cells – but in zebrafish they can also remodel the ovaries into sperm-producing testes. “It’s a pretty interesting and novel idea,” says Draper.