PGR and breast cancer: Low POLβ mRNA expression was associated with young patient age (p = 0.001), premenopausal status (p = 0.015), high tumour grade (p < 0.001), negative hormonal status (ER&PR) (p < 0.001), positive HER2 status (p < 0.001) and basal-like breast cancer molecular subtype (p < 0.001) (Supplementary Table S4).