This non-interventional study is an observational cohort with parallel groups aiming primarily to describe the success of biosimilar of ustekinumab in Behçet's syndrome in whom conventional approaches have failed or are not suitable well tolerated, and then to compare with patients receiving apremilast within routine care.

Ustekinumab previously prescribed subcutaneously at 90 mg on Week 0, 4, 12 and 20 within the standard of care.

Following non-opposition to participate, patients data will be collected, which will comprise data of the 3-month interval medical visits, except for the first month of treatment, in which the short-term tolerance of treatments is usually assessed (ie, baseline visit, then week 4, 12, 24, 36 and 52). Clinical examination, biological tests and relevant clinical scores (BDCAF, BSAS and PhGA) data that were performed within routine care. No changes to patients' usual care will be made (no additional visits, additional examinations or questionnaires), their safety and well-being remaining therefore unchanged. Data will be collected from the participant's medical record (containing medical reports and examinations, biological tests, nursing records, etc.), for the period of participation in the research, with the only purpose of meeting the objectives of the research. Data will be collected using an electronic 'eCRF observation book on the REDCap platform. The following data will be collected: demographic data (age, sex, weight, height); clinical data (history of the disease, pathology diagnosed, activity of the pathology), treatments, biological data, adverse events. No genetic data will be collected as part of the study. No data will be transferred abroad. No additional questionnaires, examinations or visits will be added by the research. Activity indexes for BS will be calculated as part of routine care. The number, duration and intensity of oral ulcers of patients will be verified by the nurse or the site investigator according to the routine care.

Patients' data with active mucocutaneous Behçet's manifestations with an indication of starting apremilast within standard of care (according to AMM and PNDS) will be retrospectively collected in order to establish a retrospective cohort for comparative purposes.

This non-interventional study is an observational and comparative cohort aiming to describe the success of biosimilar of ustekinumab in Behçet's syndrome in whom conventional approaches have failed or are not suitable well tolerated. Data on 208 patients is expected to be identified as follows:

• data of 104 patients will be collected from patients who have received the ustekinumab
• data of 104 patients will be collected from patients who have received the apremilast.

Ustekinumab is usually prescribed subcutaneously at 90 mg on Week 0, 4, 12 and 20 within the standard of care.

Following non-opposition to participate, patients data will be collected retrospectively according to local clinical practices and usual care, which usually comprises 3-month interval medical visits, except for the first month of treatment, in which the short-term tolerance of treatments is usually assessed (ie, baseline visit, then week 4, 12, 24, 36 and 52). Available data on clinical examination, biological tests and relevant clinical scores (BDCAF, BSAS and PhGA) that are performed within routine care will be retrieved. No changes to patients' usual care will be made (no additional visits, additional examinations or questionnaires), their safety and well-being remaining therefore unchanged. Data will be collected from the participant's medical record (containing medical reports and examinations, biological tests, nursing records, etc.), for the period of participation in the research, with the only purpose of meeting the objectives of the research. Data will be collected using an electronic eCRF observation book on the REDCap platform. The following data will be collected: demographic data (age, sex, weight, height); clinical data (history of the disease, pathology diagnosed, activity of the pathology), treatments, biological data, adverse events. No genetic data will be collected as part of the study. No data will be transferred abroad. No additional questionnaires, examinations or visits will be added by the research. Activity indexes for BS are usually calculated as part of routine care. The number, duration and intensity of oral ulcers of patients might be verified by the nurse or the site investigator according to the routine care.

Patients data with active mucocutaneous Behçet's manifestations with an indication of starting apremilast within standard of care will be also retrospectively collected in order to establish a comparative group, using the same parameters above mentioned

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

Two groups will be established in this observational cohort:

1. Patients with active mucocutaneous manifestations of Behçet's syndrome that are either relapsing, refractory or intolerant to conventional treatments, that have received an indication of ustekinumab according to the standard of care of tertiary centres.
2. Patients with active mucocutaneous manifestations of Behçet's syndrome treated by apremilast within standard of care will be also retrospectively collected in order to establish a comparative group.