A possible modulation of the Notch pathway by ERβ is especially interesting as Notch is a known regulator of breast development and maintenance of breast stem cells [59]; alterations in the Notch pathway have been involved in breast carcinogenesis, and in particular the Notch pathway has been implicated in the development of triple negative BC (TNBC), a particularly aggressive form of BC which does not express ERα, progesterone receptor (PR) or HER2, and which has shown resistance to all known therapies [60]. This evidence concerns the gene PGR and breast cancer.