As T-DM1 binds to HER2, theoretically, T-DM1 is more likely to kill a PIK3CA-mutated tumor cell with co-occurring HER2 amplification versus a HER2-negative, PIK3CA-mutated cell, despite the fact that the tumor as a whole is HER2-positive and PIK3CA-mutated. This evidence concerns the gene ERBB2 and neoplasm.