Nine/31 (29%) patients had monogenic disease mimicking BD: five cases of Haploinsufficiency of A20 with novel TNFAIP3 variants (p.T76I, p. M112Tfs*8, p. S548Dfs*128, p. C657Vfs*14, p. E661Nfs*36); one case of ISG15 deficiency with a novel nonsense variant (ISG15: p.Q16X) and 1p36.33 microdeletion; one case of common variable immune deficiency (TNFRSF13B: p.A181E); and two cases of TNF receptor-associated periodic syndrome (TNFRSF1A: p.R92Q). This evidence concerns the gene ISG15 and Behcet disease.