Semi-automated, inexpensive synthesis of genes from oligos made on chips. Hundreds of thousands of different oligos are synthesized on a chip, released, and amplified by PCR (PCR primers are shown by the thickest lines). Construction oligos are cut (thinnest lines) with type IIS restriction endonucleases to release internal doubled-stranded hybrids, some of which contain nucleotide base errors (red stars). Only the correct construction oligos hybridize well with complementary selection oligos (purple) immobilized via biotin (B) binding to avidin. Eluted oligos are assembled by PCR. Base errors (yellow stars) are further reduced by sequestration with MutS protein (green).