text: <unk> s <unk> s) are a sub-population of<unk>  cells (found within tumors or hematological<unk> s) that possess characteristics normally associated with stem cells. These cells are believed to be tumorigenic (tumor-forming), in contrast to the bulk of<unk>  cells, which are thought to be non-tumorigenic.<unk> s have stem cell properties such as self-renewal and the ability to differentiate into multiple cell types. A theory suggests such cells persist in tumors as a distinct population and cause relapse and metastasis by giving rise to new tumours. Development of specific therapies targeted at<unk> s holds hope for improvement of survival and quality of life of<unk>  patients, especially for sufferers of metastatic disease.
  File:Cancer stem cells text resized.svg
  Existing<unk>  treatments were mostly developed on animal models, where therapies able to promote tumor shrinkage were deemed effective. However, animals could not provide a complete model of human disease. In particular, in mice, whose life spans do not exceed two years, tumor relapse is exceptionally difficult to study.
  The efficacy of<unk>  treatments are, in the initial stages of testing, often measured by the amount of tumour mass they kill off.  As<unk> s would form a very small proportion of the tumour, this may not necessarily select for drugs that act specifically on the stem cells.  The theory suggests that conventional chemotherapies kill differentiated or differentiating cells, which form the bulk of the tumor but are unable to generate new cells. A population of<unk> s, which gave rise to it, could remain untouched and cause a relapse of the disease.
option_0: Cancer stem cell
option_1: Stem cell
option_2: Cancer Cell
option_3: Immunoblast
option_4: Reactive lymphocyte
option_5: Hematopoietic stem cell
option_6: Fibroblast
option_7: Osteoblast