1. An in vitro method of isolating an enriched or purified population of human motor neurons from a population of human embryonic stem cells comprising: providing a population of human embryonic stem cells; culturing the population of human embryonic stem cells under conditions effective to promote neuralization and embryoid body formation; administering retinoic acid and sonic hedgehog to the cultured population of human embryoid bodies in an amount effective to induce the production of motor neurons from the population of human embryoid bodies; introducing a nucleic acid molecule encoding a marker protein operably linked to a heterologous basal promoter and a mouse 3.6 kb XhaI-SseI Hb9 enhancer region into the population of human embryoid bodies which have been subjected to said administering, wherein the enhancer region selectively directs expression of the marker protein to human motor neurons, is isolated from the 9 kb Hb9 promoter region, and is placed upstream of said heterologous basal promoter; allowing the motor neurons to express the marker protein; and separating the cells expressing the marker protein from the population of human embryoid bodies which have been subjected to said inducing, whereby an enriched or purified population of human motor neurons is isolated.