The 3D graphic depicts a cut-away section of a multi-cellular tumour spheroid, based on experimentally validated mathematical models; the green region depicts well-oxygenated and proliferating tumour cells and hypoxic cells (oxygen tension < 10 mmHg) are shown in the red. Where oxygen can diffuse no further, central anoxia develops and this is seen in the image as a grey core. See R. Soc. open sci. (2014), 1, 140080 (doi:10.1098/rsos.140080).