Immunohistochemistry (IHC) is currently the first-line test for mismatch repair deficiency (MMR-D). Bou Farhat et al. show that mismatch repair (MMR) mutation signature by next-generation sequencing is a highly sensitive assay capable of detecting MMR-D cases that are missed in 1% and 5% of patients with MMR-D colorectal cancer (CRC) and endometrial cancer (EC), respectively. Patients with MMR-D tumors missed by IHC have similar clinical outcomes to patients with MMR-D by both IHC and mutation signature., Competing Interests: Declaration of interests D.J.K. reports receiving grant support from Genentech, Revolution Medicines, and AADI and serving as a consultant for Genentech, AADI, Guidepoint, Bridgebio, William Blair, and Slingshot Insights. A.H.N. receives honoraria from OncLive, TEMPUS, and Korean Society for Medical Oncology and consulting fees from Guidepoint global. L.M.S. reports serving as a consultant for Genentech, Lilly, and AstraZeneca and receiving grant funding from Genentech and Bristol Myers Squibb. E.B.F. reports serving as a clinical research coordinator for AADI., (Copyright © 2023 Elsevier Inc. All rights reserved.)