Jaw1 is a novel lymphoid-restricted gene that is expressed in a developmentally regulated fashion in both the B and T cell lineages. Jaw1 mRNA is abundantly expressed in pre-B and B cell lines with minimal or undetectable expression in plasma cell lines. Pre-T cell lines and normal mouse thymocytes express high levels of Jaw1 mRNA, whereas most mature T cell lines express low levels. Comparison of the mouse and human genes reveals that Jaw1 encodes a 539 amino acid protein with a highly conserved coiled-coil domain in the middle third of the protein and a COOH-terminal transmembrane domain. Jaw1 was localized to the endoplasmic reticulum (ER) of lymphocytes by indirect immunofluorescence and confocal microscopy. When overexpressed in HeLa cells, Jaw1 protein targeted to the ER. In vitro translation of Jaw1 in the presence of canine microsomes demonstrated that Jaw1 is an integral membrane protein of the ER and is oriented on the ER membrane facing the cytosol. Jaw1 is a member of a class of proteins with COOH-terminal hydrophobic membrane anchors and is structurally similar to proteins involved in vesicle targeting and fusion. These findings suggest that the function and/or the structure of the ER in lymphocytes may be modified by lymphoid-restricted resident ER proteins.