PD-1 Blockade in Mismatch Repair–Deficient Rectal Cancer

To the Editor: Mismatch-repair status and extended analyses of RAS mutations in patients with colorectal cancer can be predictive and prognostic.^1,2 Cercek et al. (June 23 issue)³ report a complete clinical response in 12 patients with locally advanced, mismatch repair–deficient rectal cancer after 6 months of neoadjuvant treatment with single-agent programmed death 1 (PD-1) blockade. In their study, the mismatch-repair status of the patients was determined with the use of a chromogenic immunohistochemical assay, and tumor-specific and germline comprehensive genomic analyses were performed with a next-generation sequencing assay. Although the authors report that the BRAF V600E mutation was not detected . . .