Although T7 Endonuclease I has activity on mismatched DNA, it is not an ideal enzyme for cleaving all single base pair mismatches in heteroduplex DNA. However, if your mismatch is known to contain C, you can use this enzyme for your assay.  Mismatch Endonuclease I does not recognize and cleave all mismatches in DNA. Mismatch Endonuclease I is ideal for cleaving T:T, G:G and T:G mismatches in DNA, but will also cleave T:I, G:I, G:U DNA mismatches, T:G and T:U DNA:RNA mismatches and will nick the thymine containing strand of T:C mismatches in DNA.