Fig. 1. Neural crest-specific Hoxa2 knockout. (A-C) In situ hybridization on wild-type (A), Hoxa2^flox/flox (B) and Hoxa2^flox/flox;Wnt1-Cre (C) whole-mount 10.5 dpc embryos using an antisense Hoxa2 probe. In C, Hoxa2 expression is selectively lost in the NC-derived mesenchyme of the second (arrow) and posterior branchial arches. (D-I) Middle ear (D-F) and hyoid (G-I) skeletal preparations from wild-type (D,G), Hoxa2^–/– (E,H) and Hoxa2^flox/flox;Wnt1-Cre (F,I) 18.5 dpc fetuses. Normal structures are indicated: MC, Meckel's cartilage; M, malleus; I, incus; T, tympanic bone; Go, gonial bone; Hy, hyoid bone, with lesser (LH) and greater (GH) horns. In F,I, Hoxa2^flox/flox;Wnt1-Cre mutants show an identical phenotype to the conventional Hoxa2^–/– mutants (Rijli et al., 1993), i.e. homeotic duplication of malleus (M2), incus (I2), tympanic bone (T2), partial duplication of Meckel's cartilage (MC2), transformation of gonial bone (Go*), and loss of lesser horns of the hyoid bone (asterisk in H and I).