SOX2 and breast cancer: Breast cancer stem cells can be defined by (1) expression of genes associated with ‘normal’ tissue stem cells (e.g. Notch1,2, Sox2, Pou5F1/Oct4) and with invasion and migration (e.g. Twist1,2, Vim, Snai1, Snai2) [26–30]; (2) formation of spheroid colonies in ultra-low adherence cultures [31]; (3) elevated levels of aldehyde dehydrogenases (ALDH), enzymes associated with chemoresistance, high histological tumor grade, and poor prognoses [19, 21, 32]; (4) the propensity to self-renew while spawning progenitor cells [31, 33]; and (5) an increased tumor initiation capacity in xenografts [31, 33].