ESR1 and breast cancer: Triple negative breast cancer (TNBC) represents 10-15% of all breast cancer (BC) subtypes and is defined by the lack of immunohistochemical expression of estrogen receptor, progesterone receptor and absence of overexpression and/or amplification of c-ErbB2 by immunohistochemistry (IHC) or fluorescence in situ hybridization (FISH) [1–3]