The complete sequence of the yeast genome predicts the existence of 29 proteins belonging to the ubiquitous ATP-binding cassette (ABC) superfamily. Using binary comparison, phylogenetic classification and detection of conserved amino acid residues, the yeast ABC proteins have been classified in a total of six clusters, including ten subclusters of distinct predicted topology and presumed distinct function. Study of the yeast ABC proteins provides insight into the physiological function and biochemical mechanisms of their human homologues, such as those involved in cystic fibrosis, adrenoleukodystrophy, Zellweger syndrome, multidrug resistance and the antiviral activity of interferons.