USE OF MUTATED FLP RECOGNITION TARGET (FRT) SITES FOR THE EXCHANGE OFEXPRESSION CASSETTES AT DEFINED CHROMOSOMAL LOCI

Using the FLP/FRT system for site-specific recombination and the wild- type recognition site (FRT) in conjunction with certain mutant FRT sit es, it becomes possible to provoke, with high yield, a double-reciproc al crossover event in cultured mammalian eels. It is demonstrated that this technology enables a targeting of expression cassettes to approp riate chromosomal reference sites in the recipient cell to improve the concepts of reverse genetics. The design of mutant FRT sites promotin g such a process will be delineated. Our results show that the five sp acer mutations tested are functional as the wild type but differ in th e extent of their cross-recombination, which has to be minimized for t heir simultaneous usage.