Therefore, the goal of this study was to determine the presence of MSI in a cohort of well-defined HNSCC of non-smokers and non-drinkers (NSND).<h4>Methods</h4>Clinical characteristics and tumor tissue of 119 NSND with HNSCC were retrospectively collected and analyzed for MLH1, PMS2, MSH2, and MSH6 protein expression on tissue microarrays (TMA). The gene discussed is MLH1; the disease is head and neck squamous cell carcinoma.