Breast cancer cells lacking ER protein expression, PgR protein expression and/or showing absence of HER2 protein over-expression (i.e. triple-negative breast cancer cells, basal-like) can be efficiently targeted with an anti-KAAG1 antibody and killed upon delivery of a therapeutic moiety. Antibodies and antigen binding fragments that specifically binds to KAAG1 may thus be used for the detection and therapeutic treatment of breast cancer cells that are negative for at least one of these markers. The use of antibody conjugates in the treatment of triple-negative breast cancer and/or basal-like breast cancer is disclosed herein.
