Tumors expressing ER and/or PR are termed hormone receptor positive (HR+) or luminal BC, accounting for over 70% of all BC; tumors expressing HER2 are called HER2-enriched BC, accounting for 10-20% of BC; and tumors that do not express ER, PR and HER2 are called triple negative (TN) BC, accounting for 10-15% of BC (30, 73). The gene discussed is ESR1; the disease is breast cancer.