Abstract: Binding proteins comprising a pseudoFab domain including a stabilised knockout domain and a second VH/VL that form a first functional antigen binding domain are provided. Multispecific binding proteins comprising at least one pseudoFab are also provided. Multispecific binding proteins, nucleic acids encoding binding proteins and multispecific binding proteins, expression vectors, host cells, pharmaceutical composition and methods of treatment administering the binding proteins or multispecific binding proteins described herein are also provided.