As tumors grow, they possess surrounding blood vessels and their ability to supply enough oxygen, a condition known as hypoxia. Instead of killing the tumor, however, hypoxia triggers changes in cancer cell rebound that actually affect tumor growth and spread. Better understanding hypoxia in cancer cells could help researchers find the best ways to manage the disease. This image of a mouse model for HER2-positive breast cancer uses a new imaging technique called transparent tumor tomography that visualizes the tumor microenvironment in three dimensions at single cell resolution. Hypoxic areas are in green, HER2 proteins in blue, CD31 endothelial cells in red.