PGR and breast cancer: Therefore, we selected ER-/PR-/HER2- (MDA-MB-231, MDA-MB-468) and ER+/PR+/HER2- (MCF-7, T47D) human breast cancer cell lines for further research, and we found, respectively, a low-CHK1 expression and a high-CHK1 expression cell line in each type of cancer cell (Figure S1A).