To examine the constitutive level of CYP1A1 and CYP1B1 expression in stem/progenitor cells and its counterpart non-CSCs cells, we quantified the mRNA expression levels of CYP1A1 and CYP1B1 in the mammosphere (CSCs) and adherent (non-CSCs) populations of five different breast cancer cell lines that represent different hormonal heterogeneity. Here, CYP1B1 is linked to breast cancer.