Summary of the effects of Hoxa2 overexpression on r1 derivatives The arrangement of rhombic lip derivatives in rhombomere 1 (r1) (top) and Phox2a-positive neurons (bottom) shown on a schematised flatmount of a half neural tube from normal (left) and Hoxa2-infected (right) embryos. (Top) Hoxa2 overexpression in r1 results in the loss of the EGL and an expansion of ventral rhombic lip derivatives. Ectopic trigeminal-like motor neurons are generated in caudal r1 . (Bottom) The trochlear nucleus and locus coeruleus form normally when Hoxa2 is overexpressed; however, dorsal r1 expresses high levels of ectopic Phox2a. The cells of this latter population are neither TH-positive locus coeruleus precursors that have stalled during migration nor rhombic lip derivatives that have altered their fate. egl – external granule cell layer; rl – rhombic lip.