The compelling clinical evidence of elevated IDO1 expression impacting negatively on tumour immunity and patient survival [9,10,11,12,13,14,15,16,17,18,19,20,21] has prompted many groups [22,23,24,25,26,27,28,29], to investigate novel tryptophan dioxygenase inhibitors for restoring tumour immunity. The gene discussed is IDO1; the disease is neoplasm.