 The "E. coli" strain FC40 has a high rate of mutation, and so is useful for studies, such as for adaptive mutation.Due to a frameshift mutation, a change in the sequence that causes the DNA to code for something different, FC40 is unable to process lactose.When placed in a lactose-rich medium, it has been found that 20% of the cells mutated from Lac- (could not process lactose) to Lac+, meaning they could now utilize the lactose in their environment.