We have isolated the gene encoding the Schwann cell glycoprotein P0, the major structural protein of the peripheral myelin sheath. In rats and mice, this gene is split into six exons distributed over 7 kb of DNA. The segregation of these exons is consistent with the functional segregation of the P0 protein into extracellular, membrane-spanning, and cytoplasmic domains. We find that the P0 extracellular domain is similar in structure to a single immunoglobulin variable region domain. In contrast to prototypical immunoglobulin domains, however, this P0 domain is encoded by two exons, the partitioning of which provides genetic evidence for the evolution of immunoglobulin-related domains from an ancestral half-domain. We also describe procedures for transfection of cultures of nontransformed rat Schwann cells and use these procedures to show that the Schwann cell-specific expression of the P0 gene is controlled by cis-acting elements localized upstream of exon I.