Ustekinumab in BEhçet's Syndrome STudy

Inclusion Criteria: * Age ≥ 18 years old; * Non-opposition to study; * Use of contraceptive measures; * Fulfillment of the international classification criteria for Behçet's disease, revised in 2013; * Indication for ustekinumab or apremilast within the standard of care of mucocutaneous Behçet's syndrome * For ustekinumab cohort: Active mucocutaneous manifestations of Behçet's syndrome that are recurrent, intolerant or refractory to colchicine or apremilast, including oral ulcers, genital ulcers, skin lesions (e.g., pseudofolliculitis), and/or inflammatory arthralgia/arthritis. * For apremilast cohort: recurrent active mucocutaneous manifestations of Behçet's syndrome naïve of treatment. Due to its fluctuant characteristics over time, active oral ulcers are defined as two or more oral ulcers over the month preceding inclusion and must have occurred at least three times in the previous 12-month period, despite the previous use of colchicine Exclusion Criteria: * Pregnancy; * Previous treatment failure to ustekinumab; * Contraindications to ustekinumab, such as: * Active chronic infections (e.g., active tuberculosis, replicative hepatitis B, HIV, etc.) or malignancies; * Live vaccins in the past 3 months; * Severe renal impairment (CrCl \<30mL/min/1,73m²) * Severe hepatic impairment (transaminases 5 times the upper normal values) * Severe cytopenias: Platelets \< 50 x 103/mm3 Neutrophils \< 1000/mm3 Hemoglobin \< 8 g/Dl