1. A method of isolating an enriched or purified population of motor neurons from a population of embryonic stem cells comprising: providing a population of embryonic stem cells; inducing production of motor neurons from the population of embryonic stem cells; selecting a promoter or enhancer which functions only in motor neurons; introducing a nucleic acid molecule encoding a marker protein under control of said promoter or enhancer into the induced population of embryonic stem cells; allowing the motor neurons to express the marker protein; and separating the cells expressing the marker protein from the induced population of embryonic stem cells, whereby an enriched or purified population of motor neurons are isolated.