ESR1 and breast cancer: At the constitutive level, five breast cancer cell lines that exhibit different hormonal heterogenicity, particularly SKBR-3 (HER2+), MCF-7 and T47D (ER+), and HS587T and MD-MB 231 (TNBC), were utilized to explore the differential constitutive expression of AhR-regulated genes, CYP1A1 and CYP1B1 in the mammospheres (CSCs) vs adherent (non-CSCs) cells.