ERBB2 and breast carcinoma: Lately, tumors with low levels of HER2 expression (i.e. IHC 1+ or 2+ with negative ISH), also referred to as HER2 “low” breast cancers, have been shown impressive response rates and progression-free survival (PFS) after ADC-based treatments (Iwata et al., 2018; Banerji et al., 2019; Schettini et al., 2021).