ERBB2 and breast cancer: In the ongoing DB-06 trial (NCT04494425), which is designed to evaluate the efficacy of T-Dxd in metastatic HER2-low/HR-positive metastatic BC patients with disease progression on endocrine therapy, BC patients with both HER2-low and HER2 IHC expression of >0 and <1+ (currently considered as HER2-0, or ultra-low) are included.