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Endonuclease V recognizes a broad range of aberrations in DNA such as deaminated bases or mismatches.It nicks DNA at the second phosphodiester bond 3 to a deaminated base or a mismatch.Endonuclease V obtained from Thermotoga maritima preferentially cleaves purine mismatches in certain sequence context.Endonuclease V has been combined with a high fidelity DNA ligase to develop an enzymatic method for mutation scanning.A biochemical screening of site-directed mutants identified mutants in motifs Ⅲ and Ⅳ that altered the base preferences in mismatch cleavage.Most profoundly,a single alanine substitution at Y80 position switched the enzyme to a C-specific mismatch endonuclease,which recognized and cleaved A/C,C/A,T/C,C/T and even the previously refractory C/C mismatches.