Ogura cytoplasmic male sterility (CMS) in radish (Raphanussativus) is caused by an aberrant mitochondrial gene, Orf138, thatprevents the production of functional pollen without affectingfemale fertility. Rfo, a nuclear gene that restores male fertility,alters the expression of Orf138 at the post-transcriptional level. TheOgura CMS/Rfo two-component system is a useful model for investigatingnuclear–cytoplasmic interactions, as well as the physiologicalbasis of fertility restoration. Using a combination of positionalcloning and microsynteny analysis of Arabidopsis thaliana andradish, we genetically and physically delimited the Rfo locus to a15-kb DNA segment. Analysis of this segment shows that Rfo isa member of the pentatricopeptide repeat (PPR) family. InArabidopsis, this family contains more than 450 members ofunknown function, although most of them are predicted to be targetedto mitochondria and chloroplasts and are thought to haveroles in organellar gene expression.