The study demonstrates that the Cre recombinase encoded by bacteriophage P1 can induce site-specific DNA recombination in mammalian cells. The researchers established a mouse cell line that expresses Cre under the control of the Cd2+-inducible metallothionein I gene promoter. They used two DNA substrates: a circular plasmid with two directly repeated lox sites and a pseudorabies virus (a herpesvirus) containing a lox insertion. Both substrates showed specific recombination in vivo, confirming that Cre can cause DNA synapsis and recombination in mammalian cells. This finding suggests that Cre-mediated site-specific recombination could be a useful tool for understanding and modulating genome rearrangements in eukaryotes, particularly in the context of gene regulation and chromosomal structure. The study also highlights the potential of Cre in constructing recombinant viruses and analyzing gene regulation in specific chromosomal contexts.The study demonstrates that the Cre recombinase encoded by bacteriophage P1 can induce site-specific DNA recombination in mammalian cells. The researchers established a mouse cell line that expresses Cre under the control of the Cd2+-inducible metallothionein I gene promoter. They used two DNA substrates: a circular plasmid with two directly repeated lox sites and a pseudorabies virus (a herpesvirus) containing a lox insertion. Both substrates showed specific recombination in vivo, confirming that Cre can cause DNA synapsis and recombination in mammalian cells. This finding suggests that Cre-mediated site-specific recombination could be a useful tool for understanding and modulating genome rearrangements in eukaryotes, particularly in the context of gene regulation and chromosomal structure. The study also highlights the potential of Cre in constructing recombinant viruses and analyzing gene regulation in specific chromosomal contexts.