Based on gene expression profiling, most studies categorize breast cancer into the following four major molecular subtypes: luminal A (presence of estrogen receptor – ER+ and/or progesterone receptor – PR+, and absence of HER2 expression – HER2-), luminal B (ER+ and/or PR+, HER2+), HER2 type (ER-, PR-, HER2+), and triple negative (ER-, PR-, and HER2-) (Risbridger et al., 2010). Here, ESR1 is linked to breast cancer.