Pictured is a natural killer (NK) cell that researchers developed in the lab from human pluripotent stem cells. These NK cells mimic the properties of those found in the yolk sac during the earliest stages of development. Such NK cells may be more effective as immunotherapy for cancer treatment than adult NK cells that come from bone marrow, according to a new study from Washington University School of Medicine in St. Louis. White arrows point out granules that contain potent anti-cancer enzymes. Adult NK cells have very few of these granules.