Moloney murine leukemia virus (MoMuLV)-induced rat T-cell lymphomas express discrete 1.8-, 2.2-, and 4-kb mRNA transcripts hybridizing under conditions of reduced stringency to a probe derived from a region upstream of the first exon of the Tpl-1/Ets-1 gene. Screening a cDNA library from one rat T-cell lymphoma with this genomic probe yielded 15 cDNA clones which were derived from 10 different genes. One of these genes, defined by the cDNA clone pRcT7a, was expressed as a 1.8-kb mRNA transcript in spleen and thymus but not in other normal rat tissues. Expression of the gene defined by the pRcT7a cDNA clone in a series of MoMuLV-induced rat T-cell lymphomas showed a perfect correlation with the expression of the rat leukocyte antigen MRC OX-44. Because of this observation, the pRcT7a clone was sequenced and it was shown to identify a gene coding for a 219-amino-acid protein. The homology between pRcT7a and the Tpl-1 probe used for its detection mapped within the 3' untranslated region of the pRcT7a cDNA clone. The pRcT7a protein, which exhibits four putative transmembrane regions and three putative glycosylation sites, contains a region which is nearly identical in sequence to a peptide derived from the rat leukocyte antigen MRC OX-44. This finding suggested that the pRcT7a cDNA clone defines the gene coding for OX-44. To confirm this finding, a pRcT7a construct in the retrovirus vector pZipNeo was introduced into the OX-44- T-cell lymphoma line 2788. Immunostaining with the MRC OX-44 monoclonal antibody followed by flow cytometry revealed that following gene transfer, the 2788 cells became OX-44+. Sequence comparisons revealed that pRcT7a/MRC OX-44 is a member of a family of genes which includes the melanoma-specific antigen ME491; the human leukocyte antigen CD37; the protein TAPA-1, which is expressed on the surface of human T cells and appears to be involved in growth regulation; the human gastrointestinal tumor antigen CO-029; and the Schistosoma mansoni-associated antigen Sm23.