Butyrate acts as a histone deacetylase (HDAC) inhibitor and effectively reduces GVHD by inhibiting indoleamine‐2,3‐dioxygenase (IDO)‐dependent innate immune and allo‐stimulating APC functions in a STAT‐3‐dependent manner.4, 76, 77. Here, IDO1 is linked to graft versus host disease.