Cre‐mediated germline mosaicism: a new transgenic mouse for the selective removal of residual markers from tri‐lox conditional alleles

The binary Cre‐lox conditional knockout system requires an essential part of the target gene to be flanked by loxP sites, enabling excision in vivo upon Cre expression. LoxP sites are introduced by homologous recombination, together with a selectable marker. However, this marker can disturb gene expression and should be removed. The marker is therefore often prepared with a third, flanking loxP site (tri‐lox construct), facilitating its selective removal by partial Cre‐lox recombination. We have shown that this excision can be achieved in vivo in the germline using EIIaCre transgenic mice, and have described the advantages of in vivo over in vitro removal. We show here that MeuCre40, a new transgenic mouse, more reliably and reproducibly generates an optimal partial mosaic Cre‐lox recombination pattern in the early embryo. This mosaicism was transmitted to the germline and to many other tissues. Alleles with partial deletions, in particular floxed alleles from which the selectable marker was removed, were readily recovered in the next generation, after segregation from the transgene. Segregation via paternal or maternal transmission led to successful recovery of the alleles of interest. We also obtained total deletion of the floxed regions in the same experiment, making this transgene a polyvalent Cre‐lox tool. We rigorously tested the ability of MeuCre40 to solve tri‐lox problems, by using it for the in vivo removal of neoR‐ and hprt‐expression cassettes from three different tri‐lox mutants.