Tamoxifen adminstration induces TCreERT2-mediated recombination specifically in the primitive streak or tail bud.

<p>Embryos of the indicated age carrying either the TCre (<b>A</b>) or TCreERT2 transgenes (<b>B–M</b>) were assayed for activation of the <i>R26R</i> Cre-reporter by staining for ßGal activity (blue color). TCre; <i>R26R</i> control embryos (<b>A</b>) display widespread recombination at E9.5 compared to TCreERT2;<i>R26R</i> embryos, which, 24 hours after induction, have activated R26R only in tissues recently emerged from the primitive streak at E7.5 and E8.5 (<b>B, C</b>) or tailbud at E 9.5–13.5 (<b>D–H</b>) (white arrows in <b>G</b> and <b>H</b>). An uninduced E9.5 TCreERT2;<i>R26R</i> embryo displays almost no blue cells (<b>I</b>) except for two small clusters (<b>I’</b> and <b>I”,</b> which are enlargements of the boxed regions in <b>I;</b> arrows point to individual blue cells. A similar region is circled in <b>F</b>). 48 hours after induction, TCreERT2;<i>R26R</i> embryos from E9.5–E12.5 display a recombined region that extends more rostrally (<b>J–M</b>) than in the case of a 24-hour induction period.