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This section is an MLH1 immunohistochemical stain of an endometrioid carcinoma of the uterus, taken at the time of total abdominal hysterectomy (with bilateral salping-oophorectomy).
Immunohistochemical staining is used to detect abnormalities in mismatch repair, which may reflect microsatellite instability. This pathway is important in Lynch Syndrome: a major cause of hereditary endometrial and colon cancers. In this case, the patient's endometrial carcinoma shows a deficiency in MLH1. This can be caused by either germline mutation of the MLH1 gene (a marker of Lynch Syndrome), or somatic hypermethylation of the gene promoter, leading to loss of expression. Therefore, immunohistochemistry alone cannot confirm Lynch Syndrome in this setting.