4X99:
reference: Crystal structures of immunoglobulin Fc heterodimers reveal the molecular basis for heterodimer formation., Choi HJ, Seok SH, Kim YJ, Seo MD, Kim YS, Mol Immunol. 2015 Jun;65(2):377-83. doi: 10.1016/j.molimm.2015.02.017. Epub 2015 , Mar 2. PMID: 25743157

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