Fig. 4: Examples of mismatch repair protein immunohistochemical staining variability.

In case 1 (A, B), the region without staining is likely due to poor tissue fixation (or other technical issues) as indicated by the lack of staining not only in tumor cells but also in internal control cells. In this case, the phenomenon affected all four proteins (shown here are MSH6 in A and PMS2 in B). This tumor was microsatellite stable. In case 2 (C MLH1), scattered individual glands or stretches of cells within individual glands show lack of staining, resulting in an indistinct pattern of staining heterogeneity. It is unclear what biological implications this phenomenon carries. This particular tumor was microsatellite stable and there was no evidence of germline mutation. In case 3 (D, E), the MSH6 staining (D) appears heterogeneous, with some tumor cells showing loss of staining while other tumor cells have retained staining with good intensity, but the MSH2 staining is lost (E). The etiology of the heterogenous MSH6 staining is unclear, but the phenomenon underscores the importance of using the four-antibody panel (as opposed to the two-antibody panel) in tumor testing.