Fig. 10. Fate of hindbrain motoneuron precursors in Hoxa1, Hoxb1 and Hoxb2 mutants. A schematised view of early (10.5 dpc, top row) axial identity and subsequent (12.5, bottom row) fate of hindbrain motoneuron precursors in wild type, Hoxa1, Hoxb1 and Hoxb2 embryos. At 10.5 dpc, r2-r5 have normally acquired specific identities and the corresponding motoneuron precursors do not mix. The segregated precursor populations will give rise to the trigeminal (V, derived from r2 and r3), abducens (VI, derived from r5) and facial (VII derived from r4) motor nuclei. In Hoxa1 mutants, the r3/r4 boundary does not form and most of the r5 territory is lost, resulting in a single rhombomere often referred to as rx. Some of the presumptive r4 precursors have acquired a partial r2 identity and mix with r3 precursors. The latter form an ectopic transient motor nucleus (asterisk) and sometimes a small facial (VII) motor nucleus. As a result of r5 strong reduction, the abducens motor nucleus (VIm) is lost. In Hoxb1 mutants, presumptive r4 precursors are misspecified to an r2-like identity. As a result, they all migrate in a trigeminal like manner and form an ectopic, but transient, motor nucleus (asterisk), which is cleared by cell death. In Hoxb2 mutants, some r4 precursors have acquired an r2-like identity, and as a result a transient ectopic motor nucleus (asterisk) is formed in addition to the facial motor nucleus.