A, E, F, Dorsal views of E12.5 embryos visualized live for GFP (A) or processed for Tbx20 ISH (E, F). GFP expression in r4 in a r4-Cre; ROSA26mT/mG embryo (A) labels the expression domain of Cre used for the r4-specific knockout of Celsr1 function (r4-cKO). BD, 30-μm coronal sections of an E10.5 wild type hindbrain processed for anti-GFP (green) and anti-Isl1 (red) immunostaining to mark r4-Cre expression (GFP), and FBM neurons (arrow), respectively. The light red background signal (asterisks) is due to mTomato expression in all tissues of the ROSA26mT/mG mouse where Cre is not active. Cre activity is restricted to tissues within r4 (black arrowhead). The GFP-expressing cells in D (arrow) are the earliest caudally-migrating FBM neurons originating in r4. E, F, In control embryos (r4-Cre; Celsr1fl/+ or r4-Cre; Celsr1KO/+, 20/21 embryos), Tbx20 labels trigeminal BM neurons (nV) in r2 and r3, and FBM neurons (nVII) migrating caudally from r4 to r6 (E). A small but significant number of FBM neurons (arrowheads) migrates rostrally into r3 in a majority of r4-cKO embryos (F) (r4-Cre; Celsr1KO/fl or r4-Cre; Celsr1fl/fl, 9/12 embryos).