Evolving locomotion with Hoxc9.

Limb innervation is established by opposing Hox-dependent activities. In this issue of Developmental Cell, Jung et al. (2014) show that Hoxc9 restriction of Foxp1, high levels of which specify limb-innervating motor neurons, first appeared in vertebrates concomitantly with paired appendages. Spatial control of this activity shapes neural networks controlling locomotion patterns.