A cDNA encoding the feline homologue of CD9 (fCD9) has been molecularly cloned using the procedure known as R.A.C.E. The fCD9 R.A.C.E. product displayed a high degree of homology to murine, bovine and human CD9 at both the nucleotide and amino acid level. Although the predicted amino acid sequence of fCD9 appears most similar to human CD9, it shares two characteristic features with murine and bovine CD9: human CD9 has an additional two asparagine residues in the first extracellular loop and these are not present in murine, bovine and feline CD9; feline, murine and bovine CD9 have an additional histidine residue in the second extracellular domain which is absent from both human CD9 and simian CD9 (Mitamura et al., 1992). The limited number of feline genes sequenced have shown a closer relationship to human genes than to murine genes (Fulton etal., 1987; Stewart al., 1986; O'Brien and Nash, 1982). This finding is consistent with feline CD9 being more closely related to human CD9 than other mammalian CD9s. However, the length polymorphisms common to feline, bovine, murine and rat CD9 (GenBank X76489, unpublished) which distinguish them from human and simian CD9 are an interesting contrast which suggest that these polymorphisms arose after the divergence of the primates from the other major mammalian orders.