Based on the structural plasticity of IDO1 protein that easily shifts the balance between holo- and apo-IDO1, and the recent findings suggesting a role of the non-enzymatic function of IDO1 in the tumor progression (14), in the current study we focused on the capacity of epacadostat to affect IDO1 conformations ― and thus, its enzymatic/non-enzymatic functions ― in the human ovarian cancer cell line SKOV-3, endogenously expressing human IDO1 in both conformations (19). Here, IDO1 is linked to neoplasm.