It is very important for anyone diagnosed with colorectal cancer to know whether the cancer is MSS or MSI-H. Here is a quick summary of why this matters.
In most cases, biopsy or tumor samples are examined by a pathology lab using immunohistochemistry (IHC) (they look for protein expression on slides of the biopsy or tumor).
The report that comes back may read something like this:
This means that your tumor is MSS (microsatellite stable). The statement that there is a “low probability of MSI-H” refers to the rare case where, despite having no loss of nuclear expression of MMR proteins that can be detected by IHC, there can be a defect in the gene that is detectable. For the majority of people, the statement above means they are MSS, even though the report does not explicitly state that.