The circuits that control movement are comprised of discrete subtypes of mo
tor neurons. How motor neuron subclasses develop and extend axons to their
correct targets is still poorly understood. We show that LIM homeodomain fa
ctors Lhx3 and Lhx4 are expressed transiently in motor neurons whose axons
emerge ventrally from the neural tube (V-MN). Motor neurons develop in embr
yos deficient in both Lhx3 and Lhx4, but v-MN cells switch their subclass i
dentity to become motor neurons that extend axons dorsally from the neural
tube (d-MN). Conversely, the misexpression of Lhx3 in dorsal-exiting motor
neurons is sufficient to reorient their axonal projections ventrally. Thus,
Lhx3 and Lhx4 act in a binary fashion during a brief period in development
to specify the trajectory of motor axons from the neural tube.