Despite the high degree of similarity between human and feline CD9, the predicted extracellular loops show significant amino acid variation. Significantly, fCD9 lacked a predicted site for N-linked glycosylation in the first extracellular loop: an N-X-S/T motif is conserved at this location in human, bovine and murine CD9, and in the sequence for rat CD9. A high degree of amino acid sequence variation clustered between amino acids 167 and 178 of the second extracellular loop, which is flanked by conserved cysteine residues at 165 and 179.