IDO1 and neoplasm: MSCs can migrate to the sites of inflammation and damage5 within the tumor microenvironment.6, 7, 8, 9, 10 Consistently, MSCs are often found within tumors11 and promote cancer progression and enhance metastasis.9, 12 Systemically transferred MSCs, for example, can also migrate into tumor tissue such as colon carcinomas or glioblastoma.13, 14 MSCs can act as immunoregulatory cells, releasing IL‐10 and prostaglandin E2, or by expressing the enzyme indoleamine 2,3‐dioxygenase (IDO).15, 16, 17