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).