The evolution of future anti-cancer treatments including the anti-HER3 agents under investigation might be enhanced by a robust, validated marker of HER2-HER3 dimerisation; in analogy to the potential role of EGFR-HER3 dimer in conferring resistance to cetuximab/panitumumab in the case of basal-like breast cancer patients [14]. Here, ERBB3 is linked to breast cancer.