Hence, tumors that express the estrogen receptor (ER) are denominated as luminal A and luminal B. Luminal A tumors express ERα and PR but not HER2, whereas luminal B tumors express ERα, PR, and HER2, and show a higher proliferation rate than luminal A tumors; HER2-enriched tumors lack ERα and PR expression but have high HER2 expression; and triple-negative breast cancer (TNBC) tumors do not express any of the previously mentioned receptors [49]. The gene discussed is ESR1; the disease is triple-negative breast carcinoma.