Table 1.

Categorization scheme for MMR staining interpretation

MLH1	PMS2	MSH2	MSH6	Categorization
Preserved	Preserved	Preserved	Preserved	pMMR
Lost	Lost	Preserved	Preserved	dMMR
Preserved	Lost	Preserved	Preserved
Preserved	Preserved	Lost	Lost
Preserved	Preserved	Preserved	Lost
Indeterminate or inadequate		Lost	Lost	Probably dMMR—repeat/confirm on MSI
Lost	Lost	Indeterminate or inadequate
Inadequate/indeterminate		Inadequate/indeterminate		Unclassified—repeat/send to MSI testing

pMMR proficient MMR; dMMR deficient MMR; MSI testing microsatellite instability testing by PCR