Random integration mechanism of foreign DNA by homologous illegitimate random integration and course of fad2 inserted into the X chromosome. (A) Identical foreign DNA injected into the nucleus of a eukaryotic cell is circularised and randomly cleaved by the endogenous restriction enzyme(s). It generates tandem concatemers by homologous recombination (steps 1-3, adapted from reference ). The concatemers are inserted into the host genome, mediated by homologous illegitimate random integration (HIRI), which depends on several identical nucleotide sequences (in red or blue) in the AT-rich domains on both sides, which serve as anchors for the two DNAs. Consequently, a host DNA region is replaced by the foreign DNA. During DNA replication, the repair mechanism of the cell induces foreign DNA integration into the host genome (steps 4-5). (B) Fad2 transgenic concatemers consist of a ≥322-bp complementary DNA and multiple copies inserted in a head-to-tail array by homologous recombination. During the HIRI process, 5'-TGT-3' and 5'-TTAATAG-3' (capital letters in blue) in the AT-rich domain, which exist in both foreign DNA and the X chromosome, mediate the illegitimate recombination of the transgene and the X chromosome as the homologous arms. When the homologous foreign DNA is inserted into the X chromosome, all of the 45,556 target nucleotides on the X chromosome are replaced by the foreign DNA.