Therapeutically, a class of anticancer drugs called histone deacetylase (HDAC) inhibitors are known to induce IDO expression on APCs in a signal transducer, and activator of transcription-3 (STAT3)-dependent manner; this class of medication has been used efficaciously for the prevention of GVHD in early-phase studies [46,47,48]. This evidence concerns the gene IDO1 and graft versus host disease.