Herein is reported a heterodimeric polypeptide comprising a first polypeptide comprising in N-terminal to C-terminal direction at least a portion of an immunoglobulin hinge region, which comprises one or more cysteine residues, an immunoglobulin CH2-domain and an immunoglobulin CH3-domain, and a second polypeptide comprising in N-terminal to C-terminal direction at least a portion of an immunoglobulin hinge region, which comprises one or more cysteine residues, an immunoglobulin CH2-domain and an immunoglobulin CH3-domain, wherein the first polypeptide comprises the mutations Y349C, T366S, L368A and Y407V (hole-chain) and the second polypeptide comprises the mutations S354C and T366W (knob-chain), and wherein the first polypeptide (hole-chain) comprises the mutations i) I253A or I253G, and ii) L314A or L314G or L314D, and wherein the first polypeptide and the second polypeptide are connected by one or more disulfide bridges, and wherein the CH3-domain of the first polypeptide and the CH3-domain of the second polypeptide both bind or both do not bind to protein A (numbering according to the Kabat EU index).
