Obtain a rapid burst of Cre expression in mammalian cells with footprint-free delivery of Cre mRNA in nonintegrating, virus-like particles. Cre RNA lentiviral particles (RLPs) contain no viral genome and are generated using a novel packaging technology that loads multiple biologically active mRNAs per particle. RLPs have been shown to mediate transient protein expression at high efficiency for in vitro and in vivo applications involving cell lines, primary cells, stem cells, tissues, and organisms.

Popular methods for Cre recombinase-based genome editing present tradeoffs. Transient transfection of a plasmid or mRNA limits the scope of Cre activity, minimizing the likelihood of unwanted effects associated with persistent Cre expression, but is associated with cellular toxicity, is unsuitable for in vivo applications, and is only feasible for cells that can be readily transfected. Transduction of the Cre gene sequence on a viral vector enables Cre expression in a broader range of cell types and can be applied to in vivo studies, but is associated with random genomic integration, may require activation and/or selection of target cells, and may result in undesirable persistence of Cre expression.

Cre recombinase RLPs enable researchers to overcome the above limitations and express Cre more rapidly and efficiently in mammalian cells by delivering nonviral Cre mRNA in lentivirus-derived packaging in the absence of a viral genome. In vitro studies involving RLP-mediated delivery of a luciferase reporter demonstrated that reporter expression was maximal at 4–24 hours post application and had declined rapidly at 30 hours. By contrast, expression of luciferase delivered via transfection was undetectable until 8 hours and peaked at 24 hours, while expression via lentiviral integration was undetectable until 24 hours. In an in vivo study involving a loxP mouse line (ROSA26-YFP), local injection of Cre RLPs into muscle yielded a zone of reporter gene expression that was comparable in size to a zone generated via injection of purified Cre lentivirus (Prel et al. 2015).