Through co‐culture experiments with activated T cells, we found that overexpression of SEI1 in myeloma cells can significantly inhibit the killing efficiency of activated T cells (Figure S8A–D, Supporting Information) and promote the expression of T‐cell exhaustion markers (TOX, TIGIT, and TIM3) (Figure S8E,F, Supporting Information). Here, SERTAD1 is linked to plasma cell myeloma.