Changing populations of precursors are marked with Shh-GIFM at different stages of development. (A) Alleles used for GIFM. ShhCreERmice were crossed with R26 reporter mice. The reporter gene is either lacZ or EYFP. Administration of tamoxifen (TM) to pregnant females results in Cre-mediated recombination of the reporter allele and permanent expression of the reporter gene. (B) Since TM is only stable for 24 to 36 hours, the period of Cre-mediated recombination is restricted to approximately one day after TM administration (active labeling). Labeling is retained (retained labeling) after Cre activity ceases (due to degradation of TM or downregulation of Cre expression). The retained labeling allows for the fate-mapping of recombined and marked cells. (C) X-gal whole mount staining of ShhCreER/+; R26lz/+embryos one day after the indicated time point of TM administration. (We refer to the time point of TM injection as TM followed by the embryonic day - for example, TM administration at E7.5 is TM7.5, and so on.) Black arrowheads indicate the notochord/prechordal plate, red arrowheads indicate the preoptic area, asterisks indicate the zona limitans intrathalamica (zli). (D) Summary of the distribution of Shh- expressing cells that were permanently marked at the indicated time points (TM7.5 to TM12.5) and analyzed at E12.5 or E13.5. Cells derived from Shh- expressing progenitors are initially found in the ventral midline of the posterior hypothalamic primordium (TM7.5 and TM8.5), but are later (TM9.5 to TM12.5) excluded from the midline. Shh-derived cells are also found in the preoptic area and the zli at later stages, but are almost completely excluded from the anterior hypothalamus.