Test Details: MLH1 promoter methylation is common in sporadic microsatellite unstable tumours, including colorectal cancer and endometrial cancer, and is associated with loss of MLH1 protein expression. It is rarely detected in Lynch syndrome (hereditary nonpolyposis colorectal cancer or HNPCC). Therefore, in tumours with high-level microsatellite instability (MSI-H) and/or loss of MLH1 staining by immunohistochemistry, the absence of MLH1 promoter methylation is associated with an increased likelihood of germline mutation in a DNA mismatch repair protein.