Locus coeruleus and trochlear nucleus formation is normal following global Hoxa2 overexpression (A) Flatmounted control embryo at embryonic day (E) 6, infected with RCANBP(B)Hoxa2 virus (red); Phox2a expression (black) marks the oculomotor nucleus (iii) in the midbrain, and trochlear motor nucleus (iv) and locus coeruleus (lc) in rhombomere 1 (r1). There is no Phox2a expression within the cerebellar anlage (cb). (B) Schematic diagram showing the arrangement of Phox2a-positive nuclei within r1, as shown in (A). (C) This lack of dorsal expression is confirmed in transverse section, while the locus coeruleus is clearly identified by Phox2a. (D) Hoxa2 up-regulation with RCASBP(B)Hoxa2 (red) leads to ectopic Phox2a expression in the dorsal r1/cerebellum (asterisk). (E) In a transverse section slightly caudal to that in C, ectopic expression of Phox2a is confined to the deeper cerebellar layers (asterisk), while ventral expression of Phox2a in the locus coeruleus is unaltered. (F) Hoxa2 up-regulation does not result in increased tyrosine hydroxylase (TH) staining within dorsal r1. Scale bar (A,D) = 400 μm. mb – midbrain, hb – hindbrain.