The value of this report is, therefore, not mechanistic proof but recognition of a practical morphologic-immunophenotypic observation: When a gastric carcinoma shows a sharply demarcated shift from differentiated to undifferentiated/rhabdoid morphology, dMMR should be considered, and selective ARID1A loss in the undifferentiated component may be associated with dedifferentiation. Here, ARID1A is linked to gastric carcinoma.