PPR proteins belong to large family of nucleic acid binding proteins, mainly RNA-binding proteins. Their name is defined by the presence of so-called pentatricopeptide repeat (PPR), a degenerate 35-aminoacid repeats containing from 2 up to 26 such motifs arrayed in tandem of at least in one pair. PPR motif consists of two a helices A and B forming a superhelix enclosing a groove or tunnel which is likely to be the ligand-binding site. PPR proteins are targeted mainly to mitochondria and chloroplasts where they are mainly involved in posttranscriptional processes and translation. Among PPR proteins they were also found restorer gene products which restorer pollen fertility. Some PPR proteins play roles as adaptors and partner in protein-protein interaction. PPR protein genes were discovered in all analyzed eukariotic genomes. They are especially abundant in plants.