PGR and breast cancer: In studies aimed at developing novel therapeutic techniques to combat hormone-dependent breast cancer, we found that RO decreased the expression of progesterone receptors in BT-474 and T-47D human breast cancer cells, and, furthermore, reduced levels of progestin-induced markers of cancer stem cells (CSCs), such as aldehyde dehydrogenase (ALDH), which are a hallmark of aggressive tumor growth, metastasis, and reappearance of cancer (Liang et al., 2017).