Additionally, previous studies have demonstrated an increase in functionally active IDO expression in various solid-tumor malignancies, such as breast cancer, colorectal cancer, endometrial cancer, gastric cancer, glioblastoma, gynecologic cancer, head and neck cancer, non-small cell lung cancer, small cell lung cancer, melanoma, mesothelioma, and pancreatic cancer, indicating a correlation [40,44]. This evidence concerns the gene IDO1 and melanoma.