Cancer (or malignant tumor) is a disease characterized by abnormally large cell proliferation (tumor) within normal tissue of the body, so that the survival of the latter is threatened. All these cells are derived from the same clone, a cancer initiating cell which has acquired certain characteristics which allow it to divide indefinitely. During the course of the disease, certain cells can migrate from their place of production and form metastases. For these two reasons, cancer screening should be as early as possible.