Clinical studies have shown that the levels of DENV NS1 and products of complement activation, including those with known vascular effects, such as C3a, C5a [24] and SC5b-9 [25], are present at higher levels in patients with dengue haemorrhagic fever before vascular leakage takes place, supporting the theory that complement activation is involved in the development of severe disease and unfavourable outcomes. The gene discussed is C3; the disease is dengue disease.