Positions of errors within the GFP DNA synthesis product. From bottom to top: the overlapping set of 38 oligonucleotides (thirty-six 50mers and two 5′-terminal 59mers) used to build the GFP gene and flanking sequences (arrowheads indicate the 3′-terminus of each molecule); the four intermediate assembly products used for the first round of error depletion; positions of errors present in the error-enriched (EE, yellow triangles), untreated (UN, red diamonds), error-depleted (ED1, blue circles), and twice depleted (ED2, purple squares) gene synthesis products. Per-base error rates for each of these sets are also indicated.