Finally, at least one monogenic disease associated with early-onset polyarteritis nodosa such as ADA2 (Adenosine deaminase 2) deficiency has recently been described [39] and other monogenic disorders corresponding to some cases of childhood-onset AAV with a narrower range of clinical features and a specific drug-response pattern may be discovered in the near future. Here, ADA2 is linked to polyarteritis nodosa.