When treating cancer, immunotherapy drugs known as checkpoint inhibitors can be effective by helping the immune system destroy cancer cells, but they don’t work for everyone. Prior studies suggest that an FMT from people who respond to these drugs to the guts of those who don’t can be beneficial. “The microbiome is a strong regulator of host immunity, so we hypothesise that altering it can boost immunity to help kill cancer,” says Gianluca Ianiro at the Catholic University of the Sacred Heart in Rome, Italy.