Like all ciliates, P. tetraurelia rearranges its genome at each sexual generation during the development of its somatic macronucleus from the germinal micronucleus. The rearrangements include the precise excision of short transposon-derived sequences called IES (Internal Eliminated Sequences). Excision fidelity is essential for half of the paramecium genes that contain at least one IES. The excision of a fraction of them depends on small maternal...