Adoptive T cell therapy is a type of immunotherapy that relies on identifying killer T cells that react to mutant proteins from a patient’s own tumor. Unfortunately, the mutant proteins that have been targeted in this way are not shared among tumors from different individuals, making this approach difficult to apply widely in the clinic. Now, researchers have identified a mutant tumor antigen, called GATA3, that is common among 5 to 7 percent of all breast cancer patients—about 10,000 to 15,000 women and men each year in the United States alone. Josh Lauring received a Swim Across America grant to test T cells from healthy donors for their ability to specifically recognize and destroy breast cancer cells with GATA3 mutant proteins. This proof-of-principle could lead to new immunotherapy treatments for hormone receptor-positive breast cancer.