The maxicell system (1 ) is based on the following observation: The number of lesions induced with UV light in a DNA molecule is proportional with the size of the molecule and the dose of the UV light. Since most plasmids are less than 1/1000 the size of the chromosome and are usually present in 10–100 copies per cell, it is possible to irradiate a culture of a plasmid-containing strain in such a way that every single chromosome is damaged at least once, whereas at the same time a substantial number of plasmid molecules remain undamaged.
