ESR1 and breast cancer: However, given the heterogeneity that exists among breast cancer molecular subtypes based on the ER, progesterone receptor (PR) and HER2 status, which modulate many growth factor signaling pathways such as type 1 insulin-like growth factor receptor (IGF1R) signaling [1, 2], it is evident that singularized breast cancer targeted therapies are associated with therapeutic drawbacks such as a propensity to develop therapy resistance [3–5].