ESR1 and breast cancer: Triple negative breast cancer (TNBC), defined in the clinic by the negative expression of estrogen receptor (ER), progesterone receptor (PR), and lack of overexpression of human epidermal growth factor receptor 2 (HER2), is a heterogeneous tumor that represents ~ 14–23% of all breast cancers (BC) (Lara‐Medina et al., 2011; Martinez et al., 2013; Perez‐Rodriguez, 2015).