The Integrase (Int) site-specific recombinase of coliphage HK022 catalyzes integrative and excisive DNA recombination between two attachment (att) sites in human cells without the need to supply the accessory proteins Integration Host Factor (IHF) and Excisionase (Xis). Previous work has shown that under these conditions, reactions in cis, i.e. both att sites are located on the same chromosome, can be detected without selection. However, recombination in trans, i.e. one att site positioned on a chromosome and the other on an episomal vector, was detected only after selection. Here we show that optimization of the int-HK022 gene for human codon usage according to the GeneOptimizer software algorithm, as well as addition of accessory proteins IHF and Xis improve the recombination efficiencies in human cells, such that recombinants in a trans reaction could be detected without selection.