Primarily, MSH6-deficient tumours were frequently misclassified as microsatellite stable or microsatellite instability-low, which are both biomarkers of MMR proficiency [105,106,107], as the MSH2-MSH6 heterodimer (MutSα) of the MMR system only maintains monoNR stability and not the stability of microsatellites with longer repeat motifs [108]. The gene discussed is MSH2; the disease is neoplasm.