In the meta-analysis, we found that circulating BDNF levels did not differ between BD subtypes I and II (standardized mean difference [SMD] = 0.24, 95% CI = −1.19, 0.71, p = 0.39, number of studies [k] = 3, number of participants [n] = 645) (Figure 2). This evidence concerns the gene BDNF and Behcet disease.