Although we found that luminal A–like tumors represented only 33.7% and 42.9% of all ER-positive, HER2-negative breast cancers when we applied the 2011 and 2013 St Gallen surrogate definitions, respectively, they would represent about 50% of HER2-negative luminal tumors in breast cancer patients according to our proposed new definitions. The gene discussed is ESR1; the disease is breast carcinoma.