In the current review, MCF-7 and T47D cell lines are categorized in luminal A (ER+, PR+/–, HER2–), MDA-MB-231, MDA-MB-435, and BT-20 cell lines are categorized in TNBC (ER-, PR-, HER2-), while SKBR-3 cell line is categorized in HER2 positive (ER-, PR-, HER2+) breast cancer subtype. The gene discussed is ESR1; the disease is breast carcinoma.