Fig. 5

Mismatch repair immunohistochemistry: interpretation challenges and pitfalls. a No staining in tumor and stroma—control failure; such stain cannot be interpreted. b Tumor nuclei (arrowheads) weaker than control (arrows)—abnormal staining pattern; additional study warranted. c Cytoplasmic staining present (arrow) obscuring nuclei—consider repeat staining. d MSH6 showing weak to loss of tumor staining (arrowheads) in the setting of post neoadjuvant chemoradiation therapy—consider repeat on pre-treatment biopsy. Notice background stromal cells (arrow) show intact staining. e and f MSH6 heterogenous staining secondary to MLH1/PMS2 mutation. Area of intact staining (left side of images) juxtaposed to area of staining loss (right side of images). Notice the background lymphocytes show intact staining (arrow) despite the loss of staining in tumor nuclei (arrowhead). Original magnification, a–d; f, 400×; e, 200×