In general, breast cancer can be broken down into three major categories based on the combination of receptor expression: hormone receptor (ER or PR) positive/ERBB2 negative (HR+/ERBB2–), ERBB2 positive/hormone receptor positive or negative (ERBB2+/HR– or HR+), and triple-negative (HR–/ERBB2–) [3]. The gene discussed is ESR1; the disease is breast cancer.