A substance or reactant intended to be used together with a parent IVD to perform a specific function in an assay that is used for determining microsatellite instability [abnormal repeated sequences of deoxyribonucleic acid (DNA)] within a tumour tissue specimen. This phenomenon which is associated with defective DNA mismatch repair genes is a feature of certain cancer types including hereditary nonpolyposis colon cancer (HNPCC).