The ATP-binding cassette (ABC) gene family encodes a diverse group of transporter proteins that pump a wide variety of compounds across the membranes of cells and tissues. Several human ABC transporters are overexpressed in tumor cells that are resistant to chemotherapy drugs. We have demonstrated that the ABCA3 gene is mutated in children with neonatal respiratory failure. These patients have defects in the production of lung surfactant, a lipid rich secretion produced by alveolal type II cells. Mildly affected patients were identified that have missense mutations that may be partially functional. A new member of the ABC family has been characterized, ABCC13. This gene is a pseudogene in apes, but is a functional gene in the dog and monkey genomes. We have also characterized the ABCA14, ABCA15, and ABCA16 genes in the mouse genome. These genes are exclusively expressed in the testes. Analysis of mutations in the ABCG2 gene, an important drug transporter, reveals residues that are involved in the dimerization of the protein. ABCG2 is expressed as a transporter, and dimmers must form to generate a functional protein. Drugs that inhibit this transporter, or its dimerization could be used to inhibit the growth of cancer cells, and might be particularly useful against cancer stem cells.