Based on the expression of immunohistochemical markers such as progesterone receptor (PR), estrogen receptor (ER), human epidermal growth factor receptor-2 (HER2), and Ki-67, breast cancer can be classified into four subtypes: Luminal A, Luminal B, HER-2 positive (HER2+), and triple-negative breast cancer (TNBC), where the latter is negative for all three markers (ER, PR, HER2). Here, ESR1 is linked to breast cancer.