Many important advances in molecular biology would not have been possible wlthout the use of radioisotopes It is relatively simple to substitute a radtoacttve isotope into a nucleotide to produce a molecule that has the same biological properties as the unlabeled molecule (see Chapter 6) These molecules are mcorporated into DNA sequences by a variety of protocols Unfortunately, with the high efficiency and easy incorporation of radionuchdes comes a Pandora’s Box of difficulties The short half lives of the most commonly used nucleotides (32P,33P, 35S) necessitate that the material be freshly labeled for optimal efficiency This alone makes it difficult for scientists m developing nations to use isotopic techniques. The ability of radiation to penetrate human tissue and cause damage requires that all work be done from behind shielding material Personnel exposure to radiation must be monitored on a regular basis A problem that is now looming large for the use of radiation in the molecular-biology laboratory is the lack of disposal sites for radioactive waste. Many regulatory agencies will not issue licenses to work with radlatton if no disposal site is available