In the clinical environment, immunohistochemistry (IHC)-based determination of surrogate protein marker expression is utilized to classify breast carcinomas into four subtypes: 1) estrogen receptor (ER) and progesterone receptor (PR)-positive, and Ki67 low, 2) ER, PR, and Ki67 high, 3) HER2/neu amplified, and 4) ER-, PR-, and HER2-negative, or triple-negative breast cancer (TNBC) [7]. The gene discussed is ESR1; the disease is breast carcinoma.