Characterization of Cre-mediated cassette exchange after plasmid microinjection in fertilized mouse oocytes

DNA cassette exchange methodologies are powerful tools for rapid modificati on of defined genomic loci. Until now, most studies have focused on transfe cted cell lines that were subjected to positive and negative selection to e nrich for correctly recombined clones. This is the first report on Cre-medi ated cassette exchange (CMCE) in early mouse embryos, a nonselectable biolo gical system. A Cre expression plasmid and a replacement plasmid were intro duced into fertilized oocytes by plasmid microinjection and recombination p roducts were investigated. Both, replacement plasmid and oocyte genome carr ied heterospecific lox sites (lox511 and loxP) to direct CMCE toward integr ation. Here, we demonstrate the general feasibility of CMCE in early mouse embryos and characterize the system with respect to integration efficiency, competing reactions and parameters affecting recombination rates. genesis 27:153-158, 2000. (C) 2000 Wiley-Liss, Inc.