ESR1 and breast carcinoma: In the current review, MDA-MB-231, MDA-MB-435, MDA-MB-436, MDA-MB-468, SUM159, HCC1806, HCC1937, Hs578T, EMT6 and 4T1 cell lines are categorized as hormone-independent HER2 negative breast cancer subtypes, also known as TNBC (ER-, PR-, HER2-), while SKBR-3 and MDA-MB-453 are categorized as hormone-independent HER2 positive breast cancer (ER-, PR-, HER2+) subtypes.