Wrapping and compaction of myelin sheaths around axons require specific membrane and membrane-associated proteins. Transmembrane proteins like proteolipid protein (PLP), the peripheral myelin protein 22 (PMP-22) and P0 as well as myelin basic protein (MBP) are crucial for this process. We have isolated a rat cDNA, initially denominated NS 3, that is mainly expressed in the myelinating cells of the nervous system, the oligodendrocytes and Schwann cells. The cDNA encodes a highly hydrophobic protein of 16.8 kDa with four putative transmembrane domains. The putative NS 3 protein lacks a N-terminal hydrophobic leader sequence and has no consensus sequence for N-linked glycosylation. In contrast to PLP and PMP-22, the first and third putative transmembrane domain of the NS 3 protein contain charged amino acids, a feature which resembles the structure of gap junction proteins. Sequence analysis showed that NS 3 is the rat homolog of a human gene called MAL that was cloned from, and is expressed in various T-cell lines. Therefore, we call this gene rMAL (rat MAL). In the nervous system, the expression of rMAL, mRNA begins after birth and is highest during myelination. In situ hybridization shows that rMAL mRNA is exclusively expressed in white and gray matter oligodendrocytes in the CNS and in myelinating Schwann cells in peripheral nerves. Immunohistochemistry using a peptide-specific antibody localized the rMAL protein in the myelinated areas of the CNS and PNS. Furthermore, we demonstrate by immunoblot analysis that rMAL is a component of myelin. Its structure and distribution suggest that the rMAL protein might play an important role in compact myelin. We propose that the name rMAL protein refers to rat Myelin And Lymphocyte protein.