ERBB2 and breast cancer: Regarding T-DM1 efficacy in HER2-low breast cancer patients, the retrospective analysis of the 4,258 and 4,374 g trials showed that both PFS and ORR were significantly inferior in these patients compared to the canonical HER2+ (ORR 4.8 vs. 33.8% in the 4,258 g trial and 20 vs. 41.3% in the 4,374 g trial, while PFS 2.6 vs. 8.2 in the 4,258 g trial and 2.8 vs. 7.3 in the 4,374 g trial) (Burris et al., 2011; Krop et al., 2012).