The 2019 EULAR/ACR classification criteria then confirmed diagnosis of SLE: constitutional domain, 00; hematologic domain, thrombocytopenia and autoimmune hemolysis score, 04; neuropsychiatric domain, 00; cutaneous domain, non-scarring alopecia and oral ulcers, 02; serositis domain: pleural/pericardial effusion, 05; arthritis domain, 00; renal domain, 00; antiphospholipid antibodies domain, 02; complement protein domain: low C3 and low C4 levels, 04; highly SLE-specific antibodies domain: anti-Sm and anti-ds-DNA antibodies were positive, 06. The gene discussed is C3; the disease is systemic lupus erythematosus.