The value of comparative immunology in the study of the immune system was highlighted by the cloning of cDNAs encoding feline and canine CD4 (Dumont-Drieux et al., 1992; Norimine et al., 1992; Gorman et al., 1994). Both molecules show species-specific distribution and carry serine- and threonine-rich insertions in the second extracellular domain which may alter the function of the molecule. Although cDNAs encoding CD9 have now been cloned from several species, the normal cellular function of the molecule has yet to be determined. Inter-species comparisons of tissue distribution and amino acid sequence homology may help to elucidate the role of CD9 in the immune system.