Stool transplants have now been proven to shrink or lessen the size of cancerous tumours, as the early findings from two groups discussed at the annual meeting of the American Association for Cancer Research (AACR) show. Oncologists and medical researchers conventionally use chemotherapy, radiotherapy and surgery or any combination of the three to fight cancer. Lately, there has been some work on immunotherapy to treat cancer, but that has shown limited success. But, the faecal transplant studies, in early phases as yet, suggest that tumours topped growing, or even shrank, in some patients who didn\u2019t respond to immunotherapy initially, after stool samples from patients who did respond to the drug were introduced in their guts. This perhaps points at a strong link between gut bacteria, immunity and immune system-mediated cancer treatment. Read | Mission Impossible: How Modi government met fiscal deficit target against odds After the healthy microbiome (the microbes particular to our guts) are transferred to the patient\u2019s gut, the immunotherapy drugs (called PD-1 blockers, since they enable the immune system\u2019s attack against cancerous cells by blocking the activation of the protein PD-1 found on immunity cells whose function is to shut off T-cells\u2019 ability to ward off pathogens and other foreign invaders) are given again. Gut bacteria from relatively healthier (who responded to immunotherapy) patients could be working in a manner that elicits a stronger reaction from these drugs in the attempt to fight cancer. Although data is mixed\u2014of the two studies that investigated this phenomenon, two out of the three patients in one trial and one out of three in the other displayed positive reactions to the PD-1 drugs after the healthier gut bacteria was transferred\u2014this is nevertheless a groundbreaking discovery that can revolutionise the process through which cancer can be treated. However, obstacles still persist. The question of what specific kind of microbiome enables a more favourable response to PD-1 drugs needs to be resolved. Also, patients on antibiotics that temporarily wipe out gut microbiota can\u2019t be given the treatment, which means it will be harder to treat opportunistic infections in patients whose immune barriers are down if they are to follow this line of treatment for their cancer.