The validity of a patent may expire when the time for which it was granted has elapsed, i.e. after no more than 20 years. However, other reasons for patent expiry exist, e.g. when the patent holder fails to pay the periodic fee. The patent holder may also waive the right to the patent before the Patent Office. The regulations also provide for the expiry of a patent in the event of permanent loss of use of the invention or due to the lack of the biological material needed for it, which has become unavailable and cannot be reproduced on the basis of a description.