PGR and breast cancer: In most studies the molecular subtypes of breast cancer are divided into four major groups: luminal A (ER-positive, PR-positive and HER2-negative), luminal B (ER-positive, PR-negative and HER2 positive), HER2 positive (ER-negative, PR-negative and HER2 positive), and triple negative breast cancer (TNBC, ER-negative, PR-negative and HER2 negative) (2).