The RC::L-DTA allele is designed with a cre-dependent FLEx switch containing an eGFP sequence and an inverted tox176 attenuated diphtheria toxin subunit alpha gene (DTA*G128D) that are uniquely flanked/separated by inward-facing lox sites (both lox2272 and loxP). Under control of the endogenous Gt(ROSA)26Sor promoter/enhancer regions and the CAG hybrid promoter, widespread eGFP fluorescence is observed in the absence of Cre recombinase. Subsequent exposure to Cre recombinase that places DTA*G128D into the proper orientation to be expressed results in ablation of the cre-expressing cells. RC::L-DTA mice allow selective ablation in a tissue/cell-specific manner. The RC::L-DTA allele is uniquely suitable to abrogate any unwanted, ectopic cell death resulting from the intrinsic leakiness of the transcriptional regulatory sequences in the experimental model before Cre recombinase, and also to avoid the extreme toxicity of diphtheria toxin to affect any undesired animal cell populations after Cre recombinase.

Specifically, the donating investigator reports that in the absence of Cre recombinase, mice appear phenotypically normal with robust and ubiquitous eGFP expression when visualized by direct fluorescence in embryos and adult brain. The donating investigator reports that homozygous mice are viable with no gross abnormalities. Although homozygous females are fertile, homozygous males appear to be infertile (unknown cause), and heterozygous males have been very sub-fertile.

Several cre-mediated recombination outcomes of the RC::L-DTA allele are possible based on which lox sites are recombined. The outcomes are described below, along with some experimental considerations:
i. inversion of the RC::L-DTA allele at the lox2272 sites places the DTA*G128D sequence adjacent to the CAG promoter and into the proper orientation for expression; resulting in ablation of the cre-expressing cell type.
ii. inversion of the RC::L-DTA allele at the loxP sites places the DTA*G128D into the proper orientation for expression, but the eGFP cassette (now flanked with lox2272 sites in the same orientation) remains adjacent to the CAG promoter; resulting in continued eGFP expression and no deletion of the cre-expressing cell type.
iii. recombination of outcome ii at the lox2272 sites now deletes the eGFP cassette, placing the DTA*G128D sequence adjacent to the CAG promoter and into the proper orientation for expression; resulting in ablation of the cre-expressing cell type.
iv. inversion of outcome ii at the loxP sites reverts to the original RC::L-DTA allele; resulting in continued eGFP expression and no deletion of the cre-expressing cell type.

The two lox variants, lox2272 and loxP, are compatible only with a lox sequence identical to self; they do not recombine with each other. Although some RC::L-DTA recombination outcomes require more than one Cre recombinase pulse to achieve DTA expression, no outcomes result in deletion of the DTA cassette. Because inward-facing lox sequences result in cre-mediated inversion rather than excision, surviving cells may be able to reversibly switch back-and-forth from the non-DTA-expressing orientations as long as Cre recombinase is active in the cell. Those surviving cells (outcome ii and the original RC::L-DTA allele) may eventually recombine to a DTA-expressing orientation that results in cell death. To this point the donating investigator reports that, regardless of which inversion event occurs first, continuous exposure to Cre rapidly recombines the allele into the final state (DTA in correct orientation for expression).

The tox 176 attenuated diphtheria toxin A chain (DTA*G128D) exhibits diminished enzyme activity/toxicity compared to diphtheria toxin, but retains substantial toxicity levels for ablation of mammalian cells. RC::L-DTA has this attenuated DTA in a transcriptionally-inverted confirmation located downstream of an EGFP-polyA sequence prior to Cre recombinase. This makes RC::L-DTA mice uniquely suitable to avoid the extreme toxicity of diphtheria toxin to animal cells and abrogate any unwanted, ectopic cell death resulting from the intrinsic leakiness of the transcriptional regulatory sequences in the experimental model.