In an exploratory analysis of two phase 2 trials designed for HER2-positive BC (TDM4258 g and TDM4374 g), T-DM1 showed a lower ORR (4.8 vs. 33.8% in 4258 g, and 20 vs. 41.3% in 4374 g) and PFS (2.6 vs. 8.2 months in 4258 g, and 2.8 vs. 7.3 months in 4374 g) for HER2-negative than HER2-positive BCs [77,78]. This evidence concerns the gene ERBB2 and breast cancer.