The present invention relates to antigen binding formats for use in therapeutic treatments or diagnostic assays. The present invention relates to an antigen-binding format consisting of: —a first fusion protein wherein the CH1 constant domain of an antibody is fused i) by its N-terminal end to the C-terminal end of a variable domain of an antibody and ii) by its C-terminal end to the N-terminal end of a variable domain of an antibody and, —a second fusion protein wherein the CL constant domain of an antibody is fused by its N-terminal end to the C-terminal end of a variable domain of an antibody.