What defines a cancer stem cell?

A cancer stem cell is specifically defined as a cell within a tumor that possesses the unique ability to self-renew and generate the diverse cell types that comprise the tumor. This characteristic of self-renewal is crucial because it allows cancer stem cells to maintain the tumor's growth and propagate the cancer even after treatment. Their ability to give rise to various tumor cells makes them a focal point for understanding cancer progression and resistance to therapies.

The other options do not accurately capture the defining features of cancer stem cells. A cell that is resistant to all forms of treatment fails to encompass the broader characteristics of self-renewal and differentiation, as not all cancer cells exhibit this universal resistance. While the ability to differentiate into various tissue types relates to stem cells in general, it does not specifically tie into the unique context of cancer stem cells, which focus more on their role within tumors. Lastly, a normal cell that has mutated describes a regular cellular process leading to cancer but does not specifically define a cancer stem cell's unique functional properties within a tumor environment.