Depending on the background strain, thymic leukemia antigen (TL) can be encoded by one to three genes. This locus contains the sole gene encoding TL antigen in C57BL/6 mice. Exon 3 of the gene was disrupted by the insertion of a neomycin resistance cassette. Gene inactivation was confirmed in homozygote C57BL/6 mice by a lack of TL antigen detected by flow cytometry on intestinal epithelium. (J:142515)