Durand et al. examined the expression of four immune checkpoint molecules—TIM-3, IDO (indoleamine 2,3-dioxygenase), LAG-3, and PD-L1—in ovarian cancer (OvCa) cells of various histologic subtypes, including high-grade serous, low-grade serous, mixed, poorly differentiated, endometrioid, clear cell, and mucinous carcinoma. This evidence concerns the gene IDO1 and ovarian cancer.