Chapter 3 Clinical genetics/Fig 3.21.jpg

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(A) Poorly differentiated adenocarcinoma of colon with chronic inflammatory reaction. (B) Staining for MSH2 shows presence of protein in tumour cells and inflammatory cells. (C) Staining for MLH1 shows loss of staining in tumour cells. Chronic inflammatory cells remain positive, providing an internal control. (D) Staining for PMS2 shows loss of protein. This occurs because PMS2 is stabilized by forming a complex with MLH1, which cannot take place when MLH1 expression is lost. Chronic inflammatory cells remain positive, acting as a positive internal control.