1. A method for identifying a modulator of Hh-dependent motor neuron differentiation, comprising the steps of: (a) providing a first collection of embryonic stem cells and a second collection of embryonic stem cells; (b) contacting both collections of cells with an effective amount of retinoic acid to induce differentiation of the cells to form spinal progenitor cells; (c) activating a Hh signalling pathway in both collections of cells; (d) contacting the first collection of cells with a candidate modulator; and (e) determining if the candidate modulator in step (d) modulates motor neuron differentiation by comparing the motor neuron phenotypes of the cells in the first and second collections of cells produced by steps (b)-(d) where , if the motor neuron phenotypes of the cells in the first and second collections are different, it indicates that the candidate modulator is a modulator of Hh-dependent motor neuron differentiation.