Potassium channel tetramerization domain containing 9 (KCTD9), a pentapeptide repeat-containing protein that mimics DNA structure (21), was first identified to be highly expressed in NK cells in an animal model of viral fulminant hepatitis and in patients with hepatitis B virus-associated acute-on-chronic liver failure (22, 23). This evidence concerns the gene KCTD9 and acute-on-chronic liver failure.