Because CHD is the most severe form of viral hepatitis, both the previously reported capacity of HDV to trigger an upregulation of MHC molecules at the surface of the hepatocytes12,31 and the enhanced recruitment of Th1-polarised virus-unspecific CD4+ T cells to the liver may be central to aggravate the liver disease course and warrant further investigation. The gene discussed is CD4; the disease is animal viral hepatitis.