Certain yeasts produce toxins encoded by cytoplasmic killer plasmids and kill sibling cells which have lost the killer system. Those retaining it, survive due to an immunity gene. This selection regime is at risk when cytoplasmic DNA is captured in the nucleus, but the killer plasmids have prepared for this via an unusually high A/T bias. In case of nuclear immunity gene transcription, the polyadenylation system recognizes A/T-rich motifs, cleaves the mRNA and prevents immunity expression.