While classic PAN is the idiopathic form, in secondary or non-classical PAN, another disease manifests in a “PAN-like” fashion: infections (e.g., HBV, HCV or HIV), autoinflammatory or immunodeficiency diseases (e.g., familial Mediterranean fever; adenosine-deaminase-2-deficiency), blood dyscrasias (e.g., hairy-cell leukemia, myelodysplastic and myeloproliferative syndromes), and systemic rheumatic diseases [1••, 28, 119••]. Here, ADA2 is linked to polyarteritis nodosa.