Cancer describes an enormous spectrum of diseases that all originate from uncontrolled cellular growth. Broadly divided into benign tumors (unable to metastasize) or malignant tumors (able to invade normal tissues), cancers are further defined and classified by their cell type, tissue, or organ of origin. The discovery of tumor stem cells in a range of cancers has created opportunities for researchers to identify these rare cells in both solid tumors and hematologic cancers, as well as to investigate the role of these cells at different stages of disease