IDO1 and melanoma: The use of IDO1 inhibitors as part of a combination immunotherapy strategy with PD1/PDL1 (programmed cell death protein 1/programmed death-ligand 1) checkpoint inhibitors showed promise in early-phase studies (Siefker-Radtke et al., 2018),yet failed to demonstrate improved outcomes in advanced-stage melanoma (Long et al., 2018).