Microsatellite instability (MSI) contributes to the tumorigenesis of upper urinary tract urothelial carcinomas (UUT-UCs). In this study, we first used MLH1 and MSH2 immunohistochemistry to identify patients with loss of expression of either or both of these proteins in 132 UUT-UCs. We found a total loss of MSH2 expression in 4 patients. MSI was evaluated using 5 markers in these 4 cases. All of the tumors had high MSI (MSI-H) status. Trans-activation responsive RNA-binding protein 2, an integral component of DICER1-containing complex, was a putative target of DNA mismatch repair deficiency. Truncating mutation has been identified in gastrointestinal cancers with MSI. No previous study has evaluated the mutation status of this gene in MSI UUT-UCs. In this study, we analyze the mutation of TARBP2 in MSI-H UUT-UCs with reverse transcription polymerase chain reaction. No truncating mutations were identified in the MSI-H UUT-UCs. © American Society for Clinical Pathology.