Herein is reported a blood brain barrier shuttle module comprising a brain effector entity, a linker and one monovalent binding entity which binds to a blood brain barrier receptor, wherein the linker couples the effector entity to the monovalent binding entity which binds to the blood brain barrier receptor wherein the monovalent binding entity does not comprise the variable domains of the anti-transferrin receptor antibody 8D3 (SEQ ID NO: 01 and SEQ ID NO: 02) or of the variant anti-transferrin receptor antibody 8D3v (SEQ ID NO: 01 and SEQ ID NO: 03).