**5.1 STAT4**

Signal transducer and activator of transcription-4 (STAT4) is a transcription factor that activates gene expression involved in differentiation of naïve T cells into Th1 and Th17 cells, natural killer (NK) cells, mast cells, and dendritic cells [109–111]. The association between the BD and STAT4 gene appears to be consistent in many independent reports including Korean, Turkish, and Iranians [23, 25]. The functional studies have shown that risk allele A of STAT4 rs897200 correlates clinically with BD disease score due to increased mRNA level of STAT4 gene and expression of IL-17 [112].
