...we report a case of metastatic uterine serous carcinoma due to a germline MSH6 mutation (Lynch syndrome) which was discovered because of her tumor MMR deficiency....Tumor sequencing showed a high mutational burden and an upstream somatic mutation in the same gene, p.F1088fs....Our case demonstrated a partial response and a long-term remission from the front-line single-agent pembrolizumab in a woman with metastatic uterine serous carcinoma and Lynch syndrome due to a germline MSH6 gene mutation.