Table I.

Loss of MMR protein expression.

Protein expression lost (determined by IHC)	Interpretation (defective protein)
PMS2	PMS2
MLH1 and PMS2	MLH1
MSH6	MSH6
MSH2 and MSH6	MSH2

Due to the heterodimeric nature of the MMR proteins, loss of expression of a particular protein may in fact be due to the loss of expression of its paired protein. For example, loss of PMS2 alone indicates a defect in PMS2, whereas, when expression of both MLH1 and PMS2 are lost, this is likely due to loss of MLH1, as this results in unstable PMS2. The same is true for MSH6 and MSH2, respectively.