ESR1 and breast cancer: Expression of ALDH1A3 in breast cancers inversely correlates with the estrogen receptor status,39 consistent with our Aldefluor assay showing that expression of ALDH1A3 is elevated in ER− breast cancer cell lines such as HCC-1937, SUM149, SUM159-M1a, and MDA-MB-468 but absent in ER+ cell line such as MCF7 (Figures 1A, 1B, and 2A).