These cancers can be broken down into molecular subtypes based on their receptor expression pattern: (1) Luminal A tumors tend to express estrogen receptor (ER) and usually progesterone receptor (PR), but are negative for human epidermal growth factor receptor-2 (HER2); (2) Luminal B tumors are usually ER-, PR-, and HER2-positive; (3) HER2 enriched tumors are predominantly positive for HER2, but have little to no ER or PR expression; (4) Basal-like cancers tend to be “triple-negative breast cancers” (TNBCs) that express little to no ER, PR, or HER2 [73]. Here, ESR1 is linked to cancer.