Fig. 7. The paralogous Hox3 genes play two crucial roles in neuronal fate specification. (A,B) Along the AP axis of the hindbrain, the paralogous Hox3 genes are required to suppress the expression of Hoxb1 (green) in r6. The loss of Hox3 genes results in the ectopic expression of Hoxb1 associated with the activation of r4-like facial BMN differentiation and migration program in r6 - characteristic of a homeotic transformation. Although the Hox3 genes do not influence r4 directly, the observation that it is required to genetically suppress the r4-program in r6 ensures that r4 maintains its unique identity. The Hox3 genes thus influence the identities of at least r4, r5 and r6 during hindbrain development. (C,D) Along the DV axis of r5, the combined functions of Hoxa3 and Hoxb3 are necessary for the specification of somatic motoneuron progenitors (pSMN) of the abducens nucleus. Mutations of Hoxa3 and

Hoxb3 are associated with the ectopic expression of the more dorsal high expression of the Pax6 V2 interneuron progenitor (pV2) domain (dark green) into more ventral the pSMN domain (light green). Subsequently, V2 interneurons are ectopic in the domain normally occupied by SMNs.