Site directed mutagenesis was used to change the Arg-181 of a Cd8b1 gene fragment to a stop codon. This fragment replaced the corresponding sequence in a genomic fragment of the Cd8b1 gene. A 5 kb fragment with the human CD2 locus-controlling region downstream of the mutated gene was ligated to the genomic clone and the entire construct was injected into embryos. A founder with ~20 copies/diploid genome, designated dnlyt3, was used for analysis. "CD8b1" differs from the "CD8b2" allele normally expressed in the C57BL/6 mouse strain. Monoclonal antibodies can distinguish the endogenous protein from the transgene product in hemizygous transgenic mice.