Over the years, it has become increasingly evident that cancer is a highly heterogeneous disorder. There are a multitude of different tumor types that can affect all organs and organ systems of the body. Because of genetic variations, no 2 tumors are exactly the same. Furthermore, researchers have begun to recognize that cancer progression is also affected by the interaction between tumors and the microenvironment in which they thrive. The evolution in understanding tumor biology and genetics has helped to usher in a new era of personalized medicine in which patients receive treatments tailored to the genetic makeup and biology of their tumors.