Recombinant adeno-associated viruses (rAAVs) designed to activate transgene expression in only those cells expressing Cre-recombinase (Cre-On) are widely used to introduce optogenetics constructs into specific cell types and brain regions. In many experiments, what functionally distinguishes Cre-expressing cells from their non-Cre expressing neighbors is not fully understood. A recent paper by Saunders et al. (Sabatini lab, Harvard Medical School) published in Frontiers in Neural Circuits describes two rAAV strategies that allow for simultaneous Cre-On and Cre-Off transgene expression. One strategy (Cre-Switch) achieves differential transgene expression with a single rAAV. The second strategy introduces a Cre-Off vector (FAS), built with lox sites that do not efficiently recombine with loxP or lox2272 sites, which allow FAS rAAVs to be used simultaneously with popular Cre-On DIO (double-floxed inverted ORF) a.k.a FLEX (flip-excision) rAAVs. All Cre-On, Cre-Off, and Cre-Switch rAAV vectors in Saunders et al. are freely available from Addgene.
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