Chemotherapy has been used to cure cancer, however at the same time it kills many other cells that aren't involved with cancer. The damage done by this is life-threating. Adult stem cells are used to repair this damage through bone marrow transplant. In this form of treatment, the stem cells that form into blood cells are inserted into the bone marrow of the patient and they regenerate blood. This is extremely necessary in order for the cancer treatment to be a success, because if the blood is not regenerated the patient will die.

However, chemotherapy can shrink tumors by reducing the bulk size, but unfortunately, they often fail to target CSC's which eventually leads into relapse, then death. As seen in Figure 2.1, in the standard therapy, the CSC's are not killed.

Reasearch is ongoing in hopes of finding a more efficient technique to cure cancer using stem cells.

Researchers have also found that embryonic stem cells can be used to fight against cancer. The idea is to turn embryonic cells into “natural killer” cells that will become part of the immune system. These embryonic stem cells will be injected into the immune system, and will then develop into cancer fighting cells.

Stem cells have also been discovered to help treat brain cancer. The main form therapy used to treat brain cancer is radiation therapy. Radiation therapy is killing cancer cells through radioactive waves. It can cure brain cancer, but the side effects are deadly. The patients experience cognitive impairment, they aren't able to concentrate, and experience forgetfulness. Charles Limoli, a radiation professor, experimented on rats with brain cancer. First, he applied radiation therapy, then after two days he transplanted human stem cells. This had a great impact on the rats, as the brain cells survived and new brain cells were formed. Also, these rats didn't experience cognitive impairments due to stem cells that were transplanted.