Table 2

Patterns of MMR protein loss identified on IHC31

Pattern of expression loss on IHCLikely clinical implication
Common patterns
 MLH-1 and PMS-2 MLH-1 inactivation
MLH-1 mutation
 MSH2 and MSH-6 MSH-2 mutation
EPCAM mutation (less common)
 MSH-6 alone* MSH-6 mutation
MSH-2 mutation (less common)
 PMS-2 alone† PMS-2 mutation
MLH-1 mutation (less common)
Rare patterns
 MLH-1 alone MLH-1 inactivation
MLH-1 mutation
 MSH-2 alone MSH-2 mutation
EPCAM mutation
  • *MSH-2 can form heterodimers with MMR proteins other than MSH-6 and therefore MSH-2 expression is usually not lost when an MSH-6 mutation is present.

  • †MLH-1 can form heterodimers with MMR proteins other than PMS-2 and therefore MLH-1 expression is usually not lost when a PMS-2 mutation is present.

  • IHC, immunohistochemistry; MMR, mismatch repair.