This image represents a mixture of different types of breast cancer cells residing within a tumor. Most breast tumors are comprised of breast cancer cells that differ in their shape, size, ability to invade distant tissues and degree of aggressiveness. It is thus very important to model these processes to better understand how breast cancers evolve and how they can be treated. We have modeled these heterogeneous breast tumors in mice and this image shows various types of breast cancer cells residing within one tumor.