Here, we present a case of a patient with gastric adenocarcinoma who developed ICI-induced myositis and myocarditis within 11 days of his first treatment with programmed cell death protein 1 (PD-1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) dual immunotherapy. The gene discussed is CTLA4; the disease is myositis disease.