To create an error filter, we constructed a fusion protein between MBP (19) and the mismatch binding protein from T.aquaticus (22) with a C-terminal His 6 tag (MBP-MutS-H6). MBP-MutS-H6 was overexpressed and purified from E.coli to >95% purity (Supplementary Figure 1) . MBP-MutS-H6 immobilized on amylose resin was shown to selectively retain a 40mer heteroduplex containing a deletion mutation over wild-type homoduplex (Supplementary Figure 2) .