Chee Kin Then, MD, MSc, DPhil student, University of Oxford, Oxford, UK, describes the role of the gut microbiome in cancer therapy. Whilst a majority of research relating to the gut microbiota and cancer has been focused on colorectal cancer, there has been increased attention on the link between the gut microbiome and immune cells. Dr Then highlights how the gut microbiome can subsequently modulate tumor response to radiotherapy. This interview took place at the National Cancer Research Institute (NCRI) Festival 2021.