CD44 and cancer: CSCs have been identified through biological markers – CD44+ and CD24−13 for breast CSC, CD44+, CD24+, and epithelial-specific antigen (ESA)+14 for pancreatic CSC, and C133+15 for colorectal CSCs, through selection of a side population from cancer cells during fluorescence-activated cell sorting (FACS)16, and by deriving CSCs from spheroids in a cancer cell culture grown with specific culture medium17.