Breast cancer hypoxia. 3D transparent tumour tomography image of a breast cancer specimen, showing how the growth of the tumour has exceeded the ability of the surrounding blood vessels to provide enough oxygen, a state known as hypoxia. This triggers changes in cancer cell metabolism that promotes the growth and spread of the malignant tumour. This is a HER2-positive breast cancer (human epidermal growth factor receptor 2). Hypoxic areas are in green, HER2 proteins are in blue, and CD31 endothelial cells are in red. CD31 (platelet endothelial cell adhesion molecule-1, PECAM-1) is a glycoprotein. This image is of a mouse model of breast cancer.