IN Saccharomyces cerevisiae, HO endonuclease-induced mating-type (MAT) switching is a specialized mitotic recombination event in which MAT sequences are replaced by those copied from a distant, unexpressed donor (HML or HMR)(1,2). The donors have a chromatin structure inaccessible for both transcription and HO cleavage(1,2). Here we use physical monitoring of DNA to show that MAT switching is completely blocked at an early step in recombination in strains deleted for the DNA repair genes RAD51, RAD52, RAD54, RAD55 or RAD57. We find, however, that only RAD52 is required when the donor sequence is simultaneously not silenced and located on a plasmid. RAD51, RAD54, RAD55 and RAD57 are still required when the same transcribed donor is on the chromosome. We conclude that recombination in vivo occurs between DNA molecules in chromatin, whose structure significantly influences the outcome. RAD51, RAD54, RAD55 and RAD57 are all required to facilitate strand invasion into otherwise inaccessible donor sequences.