IDO1 and neoplasm: The ligand for c-KIT is stem cell factor (SCF), which is secreted by ovarian tumors [32,33,34] and is present at high levels in tumor ascites [35], thus the question of whether IDO expression can be induced by c-KIT signaling in tumor-associated macrophages has immediate relevance for immune regulation in the ovarian tumor microenvironment and for the pathogenesis of disease.