Simple SummaryEndometrial cancer has become increasingly common owing to the recent westernization of diet and lifestyle, with 1.7% of cancer patients dying annually. Furthermore, the 5-year endometrial cancer survival rate is barely 15%. Nectin-4 has emerged as a possible biomarker and therapeutic target. Nectin-4 is highly expressed in various cancers. However, no studies have been conducted to determine the clinical importance of Nectin-4 expression in endometrial cancer. Here, we examined 320 tissue samples from patients with endometrial cancer to determine the relevance of Nectin-4 expression in the diagnosis and prognosis of endometrial cancer. Our findings emphasize the importance of Nectin-4 as a novel diagnostic tool and screening marker for assessing endometrial cancer and improving the accuracy of approaches used to predict high-risk endometrial cancer.AbstractThe adhesion molecule Nectin-4 is a new potential therapeutic target for different types of cancer; however, little is known about its diagnosis significance in endometrial cancer (EC). We found that Nectin-4 expression was significantly higher in EC tissues than in nonadjacent normal tissue. The area under the receiver operating characteristic curve value of 0.922 indicated good diagnostic accuracy for Nectin-4 expression in EC. Furthermore, Nectin-4 expression was associated with DNA mismatch repair (MMR) protein deficiency. Notably, the high Nectin-4 expression group of patients with MSH2/6-deficient EC had shorter progression-free survival than that of the low Nectin-4 expression group. The number of lymphovascular space invasion-positive patients in groups with MMR deficiency and high Nectin-4 expression was also increased compared with that in the low Nectin-4 expression group. Bioinformatics analysis revealed that alteration in Nectin-4 and MMR genes is associated with Nectin-4 expression in EC. To the best of our knowledge, this is the first study to show that Nectin-4 expression may be a potential biomarker for EC diagnosis and that high Nectin-4 expression in MMR-deficient patients with EC can predict short progression-free survival, thus providing clues to identify patients for adjuvant therapy.