Objective Lynch syndrome(LS)are associated with germline mutations in a class of genes involved in DNA mismatch repair(MMR),including hMLH1,hPMS2,hMSH2,hMSH6,and EPCAM.The issue of LS screening is complicated.Several possible barriers to screening for hereditary LS carriers include rare access to genetic counseling and testing,lack of patients knowledge about LS,clinicians limited referrals to germline testing,the high cost of testing,fewer trained professionals in cancer genetics,and difficulties in accurately interpreting test results22.Therefore,future multidisciplinary teams should collaborate in a clinical setting so as to improve the detection status of LS in patients with newly diagnosed CRC in China.Tumor testing for MMR deficiency with immunohistochemistry(IHC)for MMR proteins in all colorectal cancer(CRC)patients is routinely assessed for identifying LS in our center.However,there are still some cases with classic LS-like cancer family histories with welldefined IHC MMR proficiency(pMMR),tumors showing high level microsatellite instability(MSI-H)based on PCR-MSI analysis,suggesting the presence of hereditary predisposition related with DNA replication and repair system.This implies that a small proportion of LS patients will be missed if tumor MMR IHC testing is adopted as the only routine prescreening method without complementary MSI testing,a choice made to avoid the added costs of MSI testing for all CRCs.Methods 4545 consecutive patients who had undergone surgery for CRCs between January 2015 and December 2017 in Chinese National Cancer Center were reviewed for pathological characteristics and tumor family histories.IHC analyses of MMR proteins,including MLH1,PMS2,MSH2 and MSH6 and BRAF V600E,were routinely performed in all CRC patients at the pathology diagnosis stage in the Department of Pathology at the National Cancer Center/Cancer Hospital,Chinese Academy of Medical Sciences.208 patients with IHC pMMR and obvious tumor family histories were further analyzed in this study.The MSI status was evaluated by MSI PCR.Germline mutations were screening by Whole Exome Sequencing(WES)and target captured sequencing in normal and matched tumor samples.DNA mismatch repair assay verified the functional effects of the MMR mutants.Results Overall,4.5%(206/4545)of patients showed obvious tumor family histories in whole CRCs cohort.3.4%(7/206)of colorectal cancer samples presented MSI-H status in a subset of IHC pMMR with tumor family histories.The 7 cases had classic LS-like cancer family histories.Interestingly,germline analysis of the selected normal and matched tumor samples of the 7 cases with MSI-H and pMMR revealed pathogenic mutation of MMR gene.We also confirmed a dominant MLH1 missense mutation showing apparently reduced DNA mismatch repair efficiency in vitro assay.Conclusions Our results demonstrated that the MMR protein with rare pathogenic missense mutations,which abrogated MMR but not protein stability,could be well expressed in tumors.Genetic testing for Lynch Syndrome of rare cases with LS-like tumor family histories and IHC pMMR should not be neglected by routine IHC MMR through integrating MSI PCR technique with Next Generation Sequencing.A comprehensive molecular testing strategy that includes LGR detection is imperative for the diagnosis of LS in a clinical laboratory setting.