According to the 2019 EULAR management recommendations for systemic lupus erythematosus (SLE), this patient first meets the diagnostic criteria for SLE: positive antiphospholipid antibodies (antinuclear antibody), positive anti-double-stranded DNA antibodies, positive anti-Smith antibodies, decreased complement component 3 and complement component 4 levels, with a total classification criteria score ≥ 10 points, confirming the diagnosis of SLE. The gene discussed is C3; the disease is systemic lupus erythematosus.