Cover photograph (Copyright © 2000, American Society for Microbiology. All rights reserved.): Expression of the Schwann cell-specific lineage marker P0 in neural crest-derived cells along the ventral nerve root of a 12.5-day-old mouse embryo. The marker was visualized as a nitroblue tetrazolium formazan precipitate following in situ hybridization of transverse sections with a digoxigenin-labeled antisense probe. The P0 gene is a direct target of the HMG-domain transcription factor Sox10. P0 expression is lost upon inactivation of Sox10, implicating Sox10 in commitment to glial cell lineages and demyelinating disease. (See related article on p. 3198.)