These conflicting results could be due to differences in experimental system (i.e. breast versus brain) or highlight non-junctional activities of Cx43 in Glioblastoma Multiforme, as we now provide evidence to show that modulation of Cx43 signaling with ACT1 effectively increases gap junctional intercellular communication in breast cancer cells, and enhances drug-induced cytotoxicity of the targeted therapies tamoxifen in the ER+ MCF7 breast cancer cell line and lapatinib in the HER2+ BT474 breast cancer cell line. This evidence concerns the gene ESR1 and breast cancer.