IDO1 may act as an immunomodulatory factor that contributes to the endometrial cancer microenvironment (Zhan et al., 2020), and endometrial cancer cells have greater IDO1 activity than noncancerous endometrial cells (Zhou et al., 2021), emphasizing the importance of determining IDO1 activity in cancerous tissues. Here, IDO1 is linked to endometrial cancer.