The results of these experiments have demonstrated for the first time that the basal expressions of CYP1A1 and CYP1B1 mRNA levels were markedly overexpressed in mammosphere more than in adherent cells of all tested breast cancer cells lines, among which, MCF-7 mammosphere cells showed the highest expression levels of CYP1A1 and CYP1B1 followed by T47D cells. This evidence concerns the gene CYP1B1 and breast cancer.