The present invention describes novel immunoglobulin compositions that co-engage at least two antigens, e.g. a first and second antigen, or, as outlined herein, three or four antigens can be bound, in some of the scaffold formats described herein. First and second antigens of the invention are herein referred to as antigen-1 and antigen-2 respectively (or antigen-3 and antigen-4, if applicable. As outlined herein, a number of different formats can be used, with some scaffolds relying combinations of monovalent and bivalent bindings.
