The top two panels show tumors produced by cancer cells. The outer ring of cells (blue) has enough oxygen to survive, but not as much oxygen reaches the inner cells. At top right, tumor cells lack the SHMT2 gene and are unable to survive in this central region, as indicated by the pink stain that marks a protein produced during cell death. At top left, the cells express high levels of SHMT2, allowing some of them to survive, indicated by the blue clusters of cells. The bottom two panels are magnifications of the central tumor regions.