The present invention relates to duocarmycin-containing antibody-drug conjugates (ADCs) for use in the treatment of human solid tumors and haematological malignancies expressing HER2, in particular breast cancer, gastric cancer, bladder cancer, ovarian cancer, lung cancer, prostate cancer, pancreatic cancer, colorectal cancer, head and neck squamous cell cancer or osteosarcoma, and acute lymphoblastic leukaemia. In particular, the present invention relates to duocarmycin-containing ADCs for use in the treatment of human solid tumors with HER2 IHC 2+ or 1+ and HER2 FISH negative tissue status. Advantageously, the present invention relates to duocarmycin-containing ADCs for use in the treatment of triple negative breast cancer (TNBC).