In addition, USP18 has enzymatic activity in cleaving the covalently conjugated 15-kDa protein encoded by interferon-stimulated gene 15 (ISG15) from its targeted substrates (136), and USP18 gene-knockout mice exhibit increased susceptibility to Salmonella typhimurium or Mycobacterium tuberculosis pathogen infections (137). The gene discussed is USP18; the disease is infection.