Studies in the past decades have revealed several classic signaling molecules involved in breast tumorigenesis, such as estrogen receptor alpha (ERα) [2], progesterone receptor (PR) [3] and human epidermal growth factor receptor-2 (HER2) [4], all of which have been identified as biomarkers for molecular classification of breast cancer and targets of individual therapy of the disease [5], [6], [7]. Here, PGR is linked to breast cancer.