Fig. 1

Mismatch repair immunohistochemistry in colorectal cancer. In a proficient tumor, intact nuclear staining of MSH2, MSH6, MLH1 and PMS2 is expected. The case depicted shows retained expression of MSH2 (a) and MSH6 (b) and concomitant losses of MLH1 (c) and PMS2 (d) (i.e., a dMMR case). Staining in background stromal cells and lymphocytes serves as positive internal control and validates the quality of the immunohistochemical reaction. dMMR: deficient mismatch repair. Original magnification 200x