DNA double stranded breaks (DSBs) pose a serious threat to genome integrity. Homologous recombination (HR) (see figure) repairs DSBs by exchanging nucleotides between identical or closely related DNA molecules. DSB-repair is a highly conserved process using similar mechanisms from yeast to man. Hence, it is possible to use model organisms to study DSB-repair and draw conclusions that are relevant for humans. Since mutations that impair DSB-repair in humans can be detrimental and cause cancer it is important to study DSB-repair. Because yeast is an excellent genetic model organism we study HR in Saccharomyces cerevisiae or Baker’s yeast.

Mating-type switching has evolved on at least three independent occasions in yeasts. S. cerevisiae, K. lactis and S. pombe all have distinct mechanisms for switching. In common for all three species is that cryptic mating type loci act as donors of mating type information, which is copied from the cryptic (non-expressed) loci into the expressed Mating type (MAT) locus. In K. lactis, the mechanism and regulation of switching is only partly understood, motivating further studies of this organism.