PGR and breast cancer: The expression level of ERα36 in breast cancer tissues was positively correlated with tumor size (P < 0.001), clinical stage (P = 0.001), histological grades (P < 0.001), lymph node metastasis (P < 0.001) and progesterone receptor (PR) expression (P = 0.024), but not with patient age (P = 0.681), ERα66 (P = 0.193) or HER2 (P = 0.147) (Supplementary information, Table S1).