Malignant tumors are characterized by “progression” and transformation. Tumor cells are genetically mutated and have the plasticity to form genetic or extragenic mutations with greater likelihood. Under the multiple pressures of the body’s immune resistance, medical treatment, and its own “dysfunctional” growth, only the more malignant components can survive and develop, and as a result, tumors show a tendency to “malignantize”. Evidence suggests that in tumors with hematogenous metastases, many early microclones of tumor cells can be found in multiple vascular beds in vivo, most of which die, with very few surviving and even fewer likely to develop into clinically visible metastatic foci. Metastatic foci differ in their biological behavior, tissue characteristics, and response to therapy compared with the original foci.