The present invention encompasses a fusion protein comprising a first Fc region and a second Fc region of a heavy chain constant region pair of an antibody (immunoglobulin), wherein at least one of the N-terminal or C-terminal ends of the first Fc region and / Wherein the first Fc region and the second Fc region are mutated in such a manner that the formation of a heterodimer is promoted in the heterodimeric Fc-fused protein wherein IL-21 is bound to the IL-21, And a heterodimeric Fc-fused protein and a heterologous duplex-fusion protein. When the antibody heavy chain constant region heterologous duplex-fusion protein according to the present invention is used, the half-life of the IL-21 contained in the heterologous duplex-fusion protein can be remarkably increased.
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