Fig. 4. Comparably low Lhx4 and Er81 expression in the facial motor nucleus of Hoxb2 mutants. Ventricular view of Lhx4 (A,B) and Er81 (C,D) expression in the hindbrain of Hoxb2 heterozygous (A,C) and mutant (B,D) embryos at 14.5 dpc. Lhx4 is normally expressed in the trigeminal (V), abducens (VI), facial (VII), nucleus ambiguus (Amb) and dorsal r4/r5-derived [(VIIb) which are likely to include the superior salivatory nucleus] motoneurons (A). Er81 is expressed only in the facial motor nucleus and the trigeminal nerve exit point. (C) Both genes define specific subpopulations within the facial nucleus (red circles in A,C). In Hoxb2 mutants, expression of both markers is diminished in the facial nucleus, but only Lhx4 is expressed ectopically in a dorsal population (compare arrows in B,D).