Neu-differentiation factor (glial growth factor) has been established as an important regulator of early Schwann cell development, and the lineage relationship between immature Schwann cells and the neural crest has been clarified by the identification of the Schwann cell precursor. Progress has been made in identifying transcription factors that control Schwann cell development and in defining molecules that positively and negatively regulate myelin differentiation pathways. The tetraspan group has emerged as a set of proteins with prominent functions in Schwann cell biology.