The CreERT2/loxP system is widely used to induce conditional gene deletion in mice. One of the main advantages of the system is that Cre-mediated recombination can be controlled in time through Tamoxifen administration. This has allowed researchers to study the function of embryonic lethal genes at later developmental timepoints. In addition, CreERT2 mouse lines are commonly used in combination with reporter genes for lineage tracing and mosaic analysis. In order for these experiments to be reliable, it is crucial that the cell labeling approach only marks the desired cell population and their progeny, as unfaithful expression of reporter genes in other cell types or even unintended labeling of the correct cell population at an undesired time point could lead to wrong conclusions. Here we report that all CreERT2 mouse lines that we have studied exhibit a certain degree of Tamoxifen-independent, basal, Cre activity. Using Ai14 and Ai3, two commonly used fluorescent reporter genes, we show that those basal Cre activity levels are sufficient to label a significant amount of cells in a variety of tissues during embryogenesis, postnatal development and adulthood. This unintended labelling of cells imposes a serious problem for lineage tracing and mosaic analysis experiments. Importantly, however, we find that reporter constructs differ greatly in their susceptibility to basal CreERT2 activity. While Ai14 and Ai3 easily recombine under basal CreERT2 activity levels, mTmG and R26R-EYFP rarely become activated under these conditions and are therefore better suited for cell tracking experiments.

中文翻译：

---

不依赖他莫昔芬的报道基因重组限制了使用CreERT2品系的谱系追踪和镶嵌分析。

CreERT2 / loxP系统被广泛用于诱导小鼠中的条件基因缺失。该系统的主要优点之一是可通过他莫昔芬的给药及时控制Cre介导的重组。这使研究人员可以在以后的发育时间点研究胚胎致死基因的功能。此外，CreERT2小鼠系通常与报告基因结合使用以进行谱系追踪和镶嵌分析。为了使这些实验可靠，至关重要的是，细胞标记方法只能标记所需的细胞群及其后代，因为报告基因在其他细胞类型中的表达不忠实，甚至在不希望的时间意外标记了正确的细胞群这可能会导致错误的结论。在这里，我们报告我们研究的所有CreERT2小鼠品系都显示出一定程度的他莫昔芬独立，基础，Cre活性。使用两个常用的荧光报告基因Ai14和Ai3，我们显示了这些基础Cre活动水平足以在胚胎发生，出生后发育和成年期标记各种组织中的大量细胞。细胞的这种意外标记给谱系追踪和镶嵌分析实验带来了严重的问题。然而，重要的是，我们发现报告基因构建体对基础CreERT2活性的敏感性差异很大。尽管在基础CreERT2活性水平下Ai14和Ai3容易重组，但在这些条件下mTmG和R26R-EYFP很少被激活，因此更适合细胞跟踪实验。