The ATP-binding cassette (ABC) transporter superfamily contains membrane proteins that translocate a wide variety of substrates across extra- and intracellular membranes, including metabolic products, lipids and sterols, and drugs. Overexpression of certain ABC transporters occurs in cancer cell lines and tumors that are multidrug resistant. Genetic variation in these genes is the cause or contributor to a wide variety of human disorders with Mendelian and complex inheritance including cystic fibrosis, neurological disease, retinal degeneration, cholesterol and bile transport defects, anemia, and drug response phenotypes. Conservation of the ATP-binding domains of these genes has allowed the identification of new members of the superfamily based on nucleotide and protein sequence homology. Phylogenetic analysis places the 48 known human ABC transporters into seven distinct subfamilies of proteins. For each gene, the precise map location on human chromosomes, expression data, and localization within the superfamily have been determined. These data allow predictions to be made as to potential function(s) or disease phenotype(s) associated with each protein. Comparison of the human ABC superfamily to that of other sequenced eukaryotes including Drosophila indicated that there is a rapid rate of birth and death of ABC genes and that most members carry out highly specific functions that are not conserved across distantly related phyla.