Transient transfection of a mixture of ERα small interfering RNAs (siRNAs) containing four different sequences of siRNA was shown to significantly reduce ERα protein levels in the ZR75-1 and MCF-7 breast cancer cells and the ERα level could be restored by co-transfection with an ERα expression plasmid (Figure 4A). Here, ESR1 is linked to breast carcinoma.