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Schematic model of CD28, CTLA4, and CTLA4‐CD28. CD28 and CTLA4 interact with the shared ligands CD80 and CD86. Upon binding to the ligand, CD28 activates T‐cell receptor (TCR) signaling, whereas CTLA4 inhibits signaling. Because ligand affinity is greater in CTLA4 than CD28, normal T cells are controlled to prevent overactivation by CTLA4. In CTLA4‐CD28, the affinity of this fusion is suspected to be identical to that of CTLA4, and the fusion gene activates downstream
