The R26^TG allele has a CMV enhancer/chicken beta-actin core promoter-driven "MADM TG" cassette inserted into the Gt(ROSA)26Sor locus on chromosome 6. The "MADM TG" cassette has an ATG start codon, a beta-globin intronic sequence (containing one frt and several lox sites), and a C-terminal portion of mut4EGFP. Mice homozygous for the R26^TG allele are viable and fertile, with no gross behavioral or observable abnormalities. Homozygous mice exhibit no fluorescent protein expression in absence of its reciprocal mutation (even if Cre or FLP recombinase is present).

The new TG cassette in R26^TG mice is compatible with any "GX cassette" (GFP^N-terminus-intron-X^ATG-less) where X^ATG-less is any gene without the start codon.

To combine MADM with the Tet-Off binary expression system, R26^TG mice must be crossed to mice harboring the R26^G-tTA2 allele (Stock No. 017909) to generate Cre or FLP recombinase-induced fluorescent labeling/tTA2-expression. A detailed description and figure of this MADM-Tet principle is available here.

For "new MADM-6" (new mosaic analysis with double markers on chromosome 6), R26^TG mice must be crossed to mice harboring a reciprocal mutation at the same locus (R26^GT mice; Stock No. 017912) to allow Cre or FLP recombinase-induced fluorescent labeling of cells. A detailed description and figure of this MADM-6 principle is available here.

For the MADM-6 design, when recombined to have the complete mut4EGFP protein, mut4EGFP fluorescence is visible without the need for immunostaining. The donating investigator also reports the "new MADM-6" design has several advantages compared to the original MADM-6 mice. Specifically, tdTomato-3Myc fluorescence is visible without the need for immunostaining. This allows direct fluorescent visualization of both GFP and tdTomato in live animals/cells: permitting genotypes of distinctly labeled cells in mosaic animals to be unequivocally determined prior to fixation and/or immunostaining. Also, "new MADM-6" contains both lox sites and frt sites; allowing the induction of MADM-labeling by either Cre or FLP recombinase introduction in cell phase G0 or G1. The donating investigator did not specifically test FLP recombinase-mediated interchromosomal recombination efficiency. Another advantage to the "new MADM-6" design is that the beta-actin intron contains a frt-flanked region of alternate lox sites. These lox versions, lox5171 and lox2272, are compatible only with a lox sequence identical to self; they do not recombine with each other or with loxP sites. These were included in an attempt to further increase recombination efficiency. However, the donating investigator has not yet performed specific comparisons to date (April 2012) to test if this new configuration results in increased recombination efficiency compared to the original MADM-6.