Using clustered regular interval short palindromic repeat-Cas9 gene editing to introduce variants into endogenous MSH2 sites in human embryonic stem cells, while eliminating wild-type genes, providing valuable information for determining pathogenic LS variants; using CRISPR/Cas9 system to introduce MSH2 mutations reported in the Lynch syndrome (LS) family into HeLa cells to study the phenotype of MMR deficiency. Here, MSH2 is linked to Leigh syndrome.