A 4.2-kb targeting vector consisting of IRES-LacZ with a neo-selectable marker was placed in between two BamHI sites found in the first exon of the gene locus. This vector replaced coding sequence for transmembrane domains 5 through 12. Northern blotting confirmed that the LacZ coding sequence was in a fusion transcript with the 5' portion of the gene's coding sequence. LacZ expression was detected in the thymic cortex and occasionally in the thymic corticomedullary junction. The N-terminal portion of the endogenous gene product, including transmembrane spans 1?4 of the protein, still remained expressed on the cell surface, as detected by flow cytometry. (J:140121)