Hybrid cultivars are used almost exclusively. A time-consuming procedure is needed to create a hybrid cultivar: Two inbred lines are established over several years and finally crossed under controlled conditions. The offspring of the first filial generation (F1) are robust, uniform and high-yielding but this effect disappears in subsequent generations. To generate hybrids quickly and effectively, the cytoplasmatic male sterility (CMS) is often exploited. This frequently occurring natural phenomenon is caused by mutations in certain parts of the DNA which result in male sterility: The affected plants are, e.g., unable to produce fertile pollen. This feature is helpful with regard to seed production because it prevents an undesired self-fertilisation of the maternal line. With plants or cultivars where CMS does not occur naturally, it can be evoked in the laboratory by using cell fusion techniques to include genetic material from other closely related species. In this way, CMS hybrids are produced.