Anti-HBc immunoglobulin (Ig)M (anti-HBc IgM) may be useful in two situations: (1) to distinguish an acute hepatitis caused by HBV from a hepatitis of different etiology in a chronic HBV-infected patient; and (2) to identify an acute hepatitis in some hepatitis B patients, particularly those with fulminant hepatitis B or HDV coinfection, where HBsAg may have been eliminated very rapidly. The gene discussed is CD40LG; the disease is hepatitis B virus infection.