The immune system identifies cells that should not be in an organism based on foreign or abnormal peptides (grey triangles) presented in the binding groove of MHC-I molecules on the surface of the cell. DFT1 cells (top) avoid the immune system of the Tasmanian devil by losing expression of their MHC-I molecules (orange and blue). DFT2 cells (bottom left) appear to be developing a similar trick, but still express a number of MHC-I molecules (shown in black) that can be recognized by the immune system. However, Caldwell et al. found that DFT2 cells are starting to lose MHC-I molecules (bottom right), which suggests that this form of tumor could soon become as pathogenic as DFT1. DFT1 and DFT2 also appear to share gene alleles of MHC-I genes (orange and blue), suggesting there may be something about these variants associated with the emergence of a contagious tumor.