This study demonstrates that mouse embryonic stem (ES) cells can be directed to differentiate into functional motor neurons (MNs) through a pathway that recapitulates in vivo neurogenesis. Inductive signals and transcription factors involved in MN generation have been identified, and the research shows that developmentally relevant signaling factors can induce ES cells to differentiate into spinal progenitor cells and subsequently into MNs. ES cell-derived MNs can populate the embryonic spinal cord, extend axons, and form synapses with target muscles. These findings indicate that inductive signals involved in normal neurogenesis can direct ES cells to form specific classes of central nervous system (CNS) neurons. The study outlines the steps involved in MN specification, including the role of retinoic acid (RA) in caudalizing neural progenitor cells and Sonic hedgehog (Shh) signaling in ventralizing these cells. The research also shows that ES cells can be directed to generate MNs through the combined action of RA and Shh signaling. ES cell-derived MNs were found to express markers characteristic of MNs, including HB9, and were able to extend axons and form synapses with target muscles in vivo. The study further demonstrates that these MNs can integrate into the spinal cord of chick embryos, showing a rostral cervical positional identity and projecting axons to appropriate muscle targets. The study also highlights the potential of ES cell-derived MNs for therapeutic applications, as they can innervate target muscle cells and form functional synapses. The findings suggest that insights into normal developmental signaling cascades can be applied to direct the differentiation of ES cells into specific classes of CNS neurons. The research provides a systematic approach to generating specific CNS neurons, which could be useful for treating neurodegenerative diseases. The study also shows that ES cell-derived MNs can survive and differentiate in vivo, indicating their potential for use in regenerative medicine. The results demonstrate that ES cells can be directed to generate functional MNs through the use of inductive signals, offering a promising avenue for the development of therapies for spinal cord injuries and motor neuron degenerative diseases.This study demonstrates that mouse embryonic stem (ES) cells can be directed to differentiate into functional motor neurons (MNs) through a pathway that recapitulates in vivo neurogenesis. Inductive signals and transcription factors involved in MN generation have been identified, and the research shows that developmentally relevant signaling factors can induce ES cells to differentiate into spinal progenitor cells and subsequently into MNs. ES cell-derived MNs can populate the embryonic spinal cord, extend axons, and form synapses with target muscles. These findings indicate that inductive signals involved in normal neurogenesis can direct ES cells to form specific classes of central nervous system (CNS) neurons. The study outlines the steps involved in MN specification, including the role of retinoic acid (RA) in caudalizing neural progenitor cells and Sonic hedgehog (Shh) signaling in ventralizing these cells. The research also shows that ES cells can be directed to generate MNs through the combined action of RA and Shh signaling. ES cell-derived MNs were found to express markers characteristic of MNs, including HB9, and were able to extend axons and form synapses with target muscles in vivo. The study further demonstrates that these MNs can integrate into the spinal cord of chick embryos, showing a rostral cervical positional identity and projecting axons to appropriate muscle targets. The study also highlights the potential of ES cell-derived MNs for therapeutic applications, as they can innervate target muscle cells and form functional synapses. The findings suggest that insights into normal developmental signaling cascades can be applied to direct the differentiation of ES cells into specific classes of CNS neurons. The research provides a systematic approach to generating specific CNS neurons, which could be useful for treating neurodegenerative diseases. The study also shows that ES cell-derived MNs can survive and differentiate in vivo, indicating their potential for use in regenerative medicine. The results demonstrate that ES cells can be directed to generate functional MNs through the use of inductive signals, offering a promising avenue for the development of therapies for spinal cord injuries and motor neuron degenerative diseases.