The cells inside these beads are engineered to produce a natural compound called interleukin-2, a cytokine that causes white blood cells to spring into action to fight cancer. These beads were first put to the test in laboratory experiments that placed them alongside tumors within a peritoneum, a membrane that forms the lining of the abdominal cavity. Here, the drug factories were shown to selectively generate a concentration of interleukin-2 within the tumors, with little to be seen elsewhere.