A characteristic of CMS mutations in plants, in contrast to the single base changes in human mitochondrial mutants (49, 140), is the presence of chimeric genes or chimeric loci; different open reading frames are joined together, or placed in proximal locations and cotranscribed with standard mitochondrial genes. Despite much progress, and the identification of several mitochondrial loci that specify CMS, the molecular basis of this defect is not understood in any plant species. Observations of altered electron transport in Petunia and toxin-mediated membrane disruption in maize plants, bacteria, and yeast expressing the maize urf13 gene product, provide clues to possible mechanisms for disruption of pollen development. Whether disruption in a particular mitochondrial function is at the root of CMS in all species, or whether defects in numerous mitochondrial activities can produce sterility, will only be revealed by further probing of physiological and biochemical defects present in CMS genotypes.