It is minimal in triple negative breast cancer (the most aggressive), followed by HER2+, luminal B HER2-, luminal B HER2+ and finally luminal A (the least aggressive) breast cancer, with the highest levels; and (2) higher Cx43 protein expression is associated to lower cancer recurrence risk in Oncotype DX-tested luminal B HER2- breast cancer tissues. The gene discussed is ERBB2; the disease is cancer.