Feline and human CD9 cDNA clones were sub-cloned into the SG5 mammalian expression vector and transfected into the human cell line HeLa along with a plasmid conferring resistance to G418. G418 resistant colonies were selected, expanded and analysed by flow cytometry. Figure 3 illustrates a comparison between a positive clone, HH5, and the parent cell line. Although both the parent cell line and clone HH5 reacted with the anti-human CD9 antibody FMC56 (Fig. 3c, 93.9% and 3f, 97.9%), only clone HH5 reacted with the anti-feline CD9 antibody vpgl5 (Fig. 3e, 77.7%) whereas the parent cell line did not (Fig. 3b, 1.8%). The parent cell line and the clone HH5 failed to react with the anti-feline CD4 antibody control (Fig. 3a, 2.4% and 3d, 1.9%). Therefore, the feline CD9 cDNA encodes a protein which can be translated, and the protein product transported to the cell surface and recognized by the anti-feline CD9 antibody vpgl5.