CMV infection occurs in patients with primary immunodeficiency disease or after intensive immunosuppressive therapy.[38] In addition, CMV infection often triggers MAS and is therefore considered one of the causes of AOSD.[39] Anti-cytomegalovirus IgM antibody or CMV-DNA titer has been reported to be significantly higher in the serum of AOSD patients, suggesting that CMV infection is involved in triggering the onset and relapse of AOSD.[40] One of the limitations of this case report is that it is unclear whether the elevation of IL-6 and IL-18 was due to active AOSD or CMV infection. Here, CD40LG is linked to inborn error of immunity.