Cre-mediated recombination coincides with serotonergic neurogenesis in the e11.5 embryo. Epifluorescence images of the e11.5 developing hindbrain of embryos, transversely sectioned, immunostained for GFP (green) and 5-HT or Nkx2.2 (red). In pdx1-creCre/0; ZegGFP/0 embryos, GFP was always expressed in or adjacent to newborn serotonergic neurons. Both serotonergic and non-serotonergic neurons expressed GFP in the rostral hindbrain (B, C, D) (rhombomere 1, r1; rhombomere 2, r2) and caudal hindbrain (ch) (F). The degree of co-expression of GFP and the serotonergic phenotype was greatest in the rostral hindbrain, with little overlap in the caudal hindbrain and sparse GFP expression in rhombomere 4 (r4) (E). GFP was not expressed in the Nkx2.2+ progenitor zone (D). GFP expression was not evident in sections from a pdx1-cre0/0; ZegGFP/0 embryo (A). Scale bars, 100 μm.