ERBB2 and breast cancer: Up to now, not a single predictive biomarker of T-DM1 efficacy in HER2- positive breast cancer has been identified: HER2 mRNA expression and mutations of the PiK3CA gene (encoding the p110α catalytic subunit of the phosphatidylinositol 3-kinase), for example, turned out to be “prognostic” but not “predictive” biomarkers in the advanced disease trials16.