IDO1 was even proposed as a surrogate marker for a more robust spontaneous antitumor immune response in some cancers [24], backed by studies evaluating the IDO1 expression in circulating tumor cells (CTCs) at baseline and after completion of chemoradiotherapy, the finding of a significant overexpression at baseline compared with the post-treatment counterparts, and that mRNA expression at baseline is associated with better survival in terms of progression-free survival. The gene discussed is IDO1; the disease is cancer.