Mice genetically engineered to express Cre recombinase under the control of a tissue- or cell type-specific promoter are powerful tools to analyze the function of a gene of interest in the target tissue or cells and, when crossed with a fluorescent reporter strain, to perform fate-mapping experiments. To generate Cre diver lines, the cre transgene is integrated into the marker gene by targeted, knock-in transgenesis. Alternatively, cre can be engineered into a bacterial artificial chromosome (BAC) encompassing most, if not all, the transcriptional determinants of the marker gene, which is then randomly inserted into the genome by transgenesis. In these genetically modified mice, Cre expression is expected to recapitulate the physiological expression pattern of the marker gene. In practice, however, the chromatin organization of the locus, and cis-regulatory sequences within the promoter, introns or at distance within the genome may lead to inappropriate spatio-temporal expression of the Cre (Schmidt-Supprian and Rajewsky, 2007; Song and Palmiter, 2018; Becher et al., 2019; Reizis, 2019).

To confirm tissue-specific expression, Cre driver mice are usually crossed with a strain carrying a gene cassette encoding a fluorescent protein, inserted into the ubiquitously expressed Rosa26 locus. The fluorescent reporter protein will be expressed upon Cre-mediated removal of the loxP-flanked stop sequence built into the cassette. Rosa26-floxed-EGFP and Rosa26-floxed-EYFP mice have been widely used to characterized cre-transgenic mice. The EGFP and EYPF reporter proteins, however, do not fluoresce brightly and thus lack sensitivity (Mao et al., 2001; Srinivas et al., 2001). The development of red-fluorescent proteins such as the tdTomato, which is far brighter than EGFP, combined with the introduction, in the Rosa26 locus, of strong promoter/enhancer sequences has improved the sensitivity of Rosa26-reporter mice (Dou et al., 2021).

To tag all conventional dendritic cells (cDC) and plasmatoid dendritic cells (pDC), the group of Boris Reizis generated a BAC-transgenic mice with the cre gene integrated within 160-Kb of the mouse cDC11c gene (CD11c-Cre, (Caton et al., 2007), B6.Cg-Tg(Itgax-cre)1-1Reiz/J). When crossed with Rosa26-floxed-EYFP reporter mice, Cre-mediated recombination, as evidenced by EYFP expression, was largely confined to the cDC linage with 96% of cDC and 86% of pDC expressing EYFP and minimal recombination in other hematopoietic cells (6% in T cells, 5% in B cells and 16% in NK cells, (Caton et al., 2007)). To gain in sensitivity we crossed the CD11c-Cre mice with mice expressing the tdTomato in the Rosa26 locus (Rosa26-floxed-tdTomato, B6.Cg-Gt(ROSA)26Sortm9(CAG-tdTomato)Hze/J) and found high expression of the tdTomato in multiple hematopoietic cells.