sentences:  A T5 exonuclease performs a chew-back reaction at the terminal segments, working in the 5' to 3' direction, thereby producing complementary overhangs.The overhangs hybridize to each other, a Phusion DNA polymerase fills in any missing nucleotides and the nicks are sealed with a ligase.However, the genomes capable of being synthesized using this method alone is limited because as DNA cassettes increase in length, they require propagation in vitro in order to continue hybridizing; accordingly, Gibson assembly is often used in conjunction with transformation-associated recombination (see below) to synthesize genomes several hundred kilobases in size.