The estrogen-dependent, ERα-positive T47D and MCF-7 cells contained low levels of Hes-6, in comparison with the more-aggressive, ERα-negative breast cancer cell lines MDA-MB-231 and SKBR3, which contained 4 to 10 times higher levels of Hes-6 mRNA (Figure 1a). Here, ESR1 is linked to breast cancer.