CD44 and cancer: They isolated a tumorigenic subset of cancer cells from human breast tumors based on the expression of the surface markers CD44+, CD24−/low and ESA+ (CD is short for cluster of differentiation, ESA is short for epithelial specific antigen).This was the first evidence for the existence of CSCs in breast cancer and they were the first to show that only the minority of breast cancer cells with a CD44+, CD24−/low and ESA+ phenotype have the ability to form new tumors in NOD/SCID mice [9].