ESR1 and breast cancer: We first examined the expression of circPGR in breast epithelial cell lines as well as different subtypes of breast cancer cell lines, and found that circPGR was expressed at a much higher level in ER-positive cells (T47D, HC1500, MCF7 and BT474) compared to breast epithelial cells (MCF10A and 184B5), HER2 (human epidermal growth factor receptor 2)-positive cells (SK-BR-3) and TNBC (triple-negative breast cancer) cells (HCC1806, HCC1937, MDA-MB-231, MDA-MB-468, SUM149PT, BT20 and HS578T) (Figure 7A).