ESR1 and breast cancer: Another recent study has shown that progesterone decreased miR-29 expression in breast cancer cells lines expressing PR and ER (T47D and BT474), resulting in the upregulation of Krüppel-like factor 4 (KLF4), a transcription factor required for the dedifferentiation into pluripotent stem cell phenotype and for the maintenance of CSCs; as a consequence, the authors observed an expansion of CK5+/CD44+ tumor-initiating cells [46].