Malignant stem cells in the gut secrete factors that promote the differentiation of neighbouring stem cells, thereby aiding the replacement of normal stem cells by those with cancer-promoting mutations. Shi Biao Chia & James DeGregori Decades of research have revealed how mutations contribute to the evolution of malignant cells and to the ultimate characteristics of a given tumour. There is growing recognition that the surrounding tissue environment affects the natural selection of these mutation-driven characteristics. Less appreciated, however, have been the effects of interactions between malignant cells and their neighbouring wild-type cells — and how, through these interactions, malignant cells shape the surrounding environment to their advantage. Writing in Nature, Yum et al., Van Neerven et al. and Flanagan et al. provide crucial insights into the competitive dynamics of cancer cells and their neighbouring cells in the intestine.