The present invention provides a protein complex comprising heterodimerizing regions HRI and HRII, each comprised of antiparallel β-strands and intervening regions wherein HRI and HRII are each interspersed fusion proteins of two human constant regions of an immunoglobulin or immunoglobulin-like proteins. The present invention also provides nucleic acid molecules comprising a sequence encoding said protein complexes and vectors comprising the nucleic acid. The present invention also provides the protein complex, the nucleic acid and the vector for use as a medicament. The present invention further provides a method of determining the amino acid sequence of HRI and/or of the amino acid sequence of HRII. The present invention also provides a method of producing amino acid chains of HRI and/or amino acid chains of HRII. The present invention further provides the protein complex for use in the N prophylaxis, treatment or diagnosis of a disorder or a disease.