MSH6 and neoplasm: Immunohistochemistry (MMR-IHC) of different sections of formalin-fixed paraffin-embedded (FFPE) blocks of the surgically-removed tumor demonstrated heterogeneity in MMR status, with one section demonstrating complete loss of MSH6 expression and evaluated as MMR deficient (dMMR) (Fig. 2a), while another section demonstrating clonal loss of MSH6 (Fig. 2b, MSH6-1X), but still evaluated as MMR proficient (pMMR) due to moderate staining in some clones (Fig. 2b).