To systematically assess potential intratumoral heterogeneity of MMR protein expression in neuroendocrine neoplasms with MMR deficiency, a cohort of 239 NETs and NECs was screened on a tissue microarray (TMA) format by IHC, followed by a large section evaluation of cancers suspected for MSI by repeated IHC and PCR analysis. The gene discussed is MRC1; the disease is neuroendocrine neoplasm.