ESR1 and breast cancer: We found that in the MCF7 and T47D cells that express high levels of ER-α36, MCF7/36 and T47D/36, the populations of CD44+/CD24− cells were significantly increased compared to the control cells transfected with the empty expression vector, suggesting that ER-α36 is involved in positive regulation of ER-positive breast cancer stem/progenitor cells (Figure 2B).