This pro-tumorigenic effect of epacadostat in the SKOV-3 tumor, together with a recently published effect of epacadostat in murine plasmacytoid DCs, combined with the observation that the non-enzymatic IDO1-mediated functions may play a key role in tumor progression of melanoma and glioblastoma (13, 14), urgently calls for a better understanding of the role of IDO1 in the TME. The gene discussed is IDO1; the disease is melanoma.