Cancer stem cells are a small subset of cells within tumors that possess stem like properties, such as the ability to self renew and to give rise to the diverse cell types that make up a cancer. They are thought to sit at the top of a cellular hierarchy, much like normal tissue stem cells, and to drive tumor initiation, progression, and relapse.

Research shows that cancer stem cells can remain quiescent, or divide slowly, which helps them evade treatments that target rapidly dividing cells. After chemotherapy or radiotherapy reduces the bulk of a tumor, surviving cancer stem cells can regenerate it, contributing to recurrence and metastasis. They also often express high levels of drug efflux pumps and DNA repair enzymes, and interact with a supportive microenvironment, all of which enhance resistance.

Cancer stem cells have been identified in blood cancers and in solid tumors such as breast, brain, colon, and pancreatic cancers, using surface markers and functional assays like their ability to form new tumors in animal models. However, markers vary between cancer types and are not always exclusive to stem cells, making identification challenging.

Current research focuses on disrupting pathways that maintain stemness, such as Wnt, Notch, and Hedgehog, altering the tumor microenvironment that protects these cells, and forcing them to differentiate into less aggressive cells. Combinations of conventional therapies with targeted approaches against cancer stem cells are being explored to achieve more durable responses and reduce relapse, while minimizing harm to normal stem cells that are essential for tissue regeneration.