Fig. 1. Wnt1-Cre recombination in the neural crest cells is marked by the expression of ß-galactosidase in Wnt1-Cre; R26R embryos. (A,B) At 4 somites, Wnt1-Cre mediates loxP recombination in the forming NCCs of the hindbrain, but not the neural tube. (C,D) Multiple neural crest cells are positive for ß-galactosidase activity in the neural tube at 6 somites. (E,F) NCCs are more plentiful at the eight-somite stage and are present at more caudal levels of the neural tube. Black arrowheads indicate the NCCs. B,D,F are dorsal, higher magnification views of embryos shown in A,C,E, respectively. Anterior is towards the top in B, towards the left in D,F. (G) A transverse section through the first pharyngeal arch (pa) of an embryo at similar stages to E,F. (H) E9.0 (15 somites) and (I) E10.5 embryos show caudal progression of Wnt1-Cre activity and increased NCC induction and migration.