The epithelial cells that line the lumen of the small intestine are constantly replenished. Tissue homeostasis is maintained by multipotent intestinal stem cells that reside between villi within the crypts. Also known as crypt cells, intestinal stem cells have the ability to self-renew and regenerate intestinal tissue by differentiating into endocrine cells, enterocytes, goblet cells, and Paneth cells. Intestinal stem cells express markers such as Lgr5/GPR49 and Telomerase Reverse Transcriptase (TERT). In addition, intestinal stem cells are thought to express low levels of SOX9, while SOX9 gain of function mutations may promote unregulated stem cell proliferation and gastrointestinal cancers.