Three landmark studies confirm that faecal microflora transplantation is a promising approach to increase the effectiveness of immunotherapy in advanced solid tumors. These studies provide insights with major implications for microbiome therapeutic development. The article was published in Nature Medicine on March 9, 2026 with doi:10.1038/s41591-026-04285-x. Modulation of the microbiome affects response to immunotherapy by blocking checkpoints such as PD-1 and CTLA-4. Studies have identified specific microbial signatures in patients' stools that predict the success of combination immunotherapy in 25% of patients with advanced rare cancers. These signatures apply across different types of cancer, such as melanoma, and across countries. In the future, they may lead to the development of probiotics as living biotherapeutics to support immunotherapy.