Is CreERT2 an unstable structure? Is it true that tamoxifen induction and then Xgal staining show only the expression site of NKX2.5 at the induction time? If we do tamoxifen induction at the late stage of development, LacZ activity would show all regions that express NKX2.5 at the late stage or at the early to late stages?
This is a common lineage tracing system which uses a ROSA26-loxp-STOP-loxp-lacZ locus (or equivalent Cre-dependent designs). After Cre-lox recombination, the stop portion will be removed leading to ubiquitous expression of lacZ by ROSA26 promoter.
Cre-ER2 is a fusion of Cre recombinase and Estrogen Receptor. This fusion protein can not carry out the recombination on genomic DNA in absence of tamoxifen (an estrogen antagonist). Tamoxifen binds to ER domain and exposes its NLS (nuclear localization signal), then the Cre-ER2 can enter into the nucleus to perform so-called Cre-lox recombination.
If you express Cre-ER2 under the control of Nkx2.5 promoter then the Cre-ER2 activity (ROSA26-lacZ rearrangement and activation) will be temporally limited to tamoxifen treatment and spatially limited to Nkx2.5+ cells. After you remove the tamoxifen, these cells will remain lacZ+ even if they become Nkx2.5- later. So, during the later stages, you can trace the lineage of the cells which were Nkx2.5+ at the stage of tamoxifen treatment.
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