Criteria include a diagnosis of non-thrombocytopenic skin purpura with lower limb predominance as the main symptom and 1 of 4 additional criteria: 1) abdominal pain, 2) arthritis or joint pain, 3) kidney involvement (proteinuria >0.3 g/24h or a urine protein/creatinine ratio >30 mg/mmol on a spot morning sample or hematuria of >5 erythrocytes/high-power field), and 4) a leukocytoclastic vasculitis with predominant IgA deposits or kidney biopsy with predominant IgA deposits. The gene discussed is CD79A; the disease is Arthritis.