This case report describes a 75-year-old female patient with deficient mismatch repair sigmoid colon adenocarcinoma who developed severe multi-organ irAEs, including myositis, hepatitis, and steroid-refractory myocarditis 25 days after receiving QL1706 (a PD-1/CTLA-4 bispecific antibody). The gene discussed is CTLA4; the disease is myositis disease.