20 In humans, however, free ISG15 stabilises USP18 and acts as a negative regulator of the IFN pathway.21,22 ISG15-null patients present with inflammation due to deregulation of the type I interferon response and, interestingly, are susceptible to mycobacterial, but not viral, diseases.21,23. The gene discussed is ISG15; the disease is glycogen storage disease VI.