Pre-made lentiviral particles bicistronically express two individual proteins: nuclear permeable CRErecombinase and the GFP marker under the same EF1a promoter. A Puromycin marker was expressed under RSV promoter. Particles allows to sort or select transduced cells for long term expression via GFP signal or via Puromycin antibiotic selection.

CRE and GFP are constitutively expressed in high level, indenpendent of cell types. GFP signal provides a convenient, realtime monitoring theparticles performance.

CRE protein, from bacteriophage P1, catalyzes recombination between 34 base pair target sequences,named lox sites. CRE lentiviral particlescancatalyzethe jointing event In Vivoin the presence oflox sites. Purified CRE enzyme can joint individual plasmids containing lox sites.