ESR1 and breast cancer: To isolate a population of breast cancer tumor-initiating cells (TICs), we stained the TNBC (HER2-/ER-/PgR-) and TPBC (HER2+/ER+/PgR+) described above with antibodies known to differentiate breast cancer stem cells, including CD24 (a cell adhesion molecule), aldehyde dehydrogenase 1 (ALDH1; an enzyme that catalyses the oxidation of aldehydes) and CD44 (a cell-surface glycoprotein involved in cell-cell interactions, cell adhesion and migration).