The invention relates to anti-HER3/HER4 antigen binding proteins, e.g. anti-HER3/HER4 antibodies, that bind to the beta-hairpin of HER3 and the beta-hairpin of HER4, methods for selecting these antigen binding proteins, their preparation and use as medicament.本發明係關於結合至HER3之β-髮夾結構及HER4之β-髮夾結構之抗HER3/HER4抗原結合蛋白(例如抗HER3/HER4抗體)、選擇該等抗原結合蛋白之方法、其製備及作為藥劑之用途。