CD44 and breast carcinoma: Since CD44+/CD24− were widely used as a cell surface marker of CSCs, we next detected the expression of CD44+/CD24− in breast cancer cells treated with morphine (10 μM) and found that the CD44+/CD24− proportion were increased from 47.87 ± 1.01% to 68.8 ± 2.68% in BT549 cells (Figure 1E).